Mercedes-AMG SL 55 vs BMW M850i: 2023 twin test review

The SL badge is one of the most revered not only in Mercedes history, but also in the sports car world as a whole. Dating back to 1954 and the beautiful 300 SL Gullwing, the halo model for the Stuttgart brand has been a style icon, with six generations of classy designs that set an industry standard in understated luxury. 

However, for this seventh generation the SL has taken a slightly different path. Mercedes has let its AMG performance division loose on the car’s development, so does that mean we’re looking at a much more hardcore, in-your-face attitude? Or has AMG gone soft and kept the qualities which made the predecessors of this SL such desirable sports cars?

Mercedes isn’t the only German brand to take the heavyweight in a smart suit approach. The BMW M850i Convertible looks understated, but there’s a powerful punch under the bonnet. We’ve brought the two together to see which is best.

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Mercedes-AMG SL 55

At £147,475, the SL 55 certainly comes with a big asking price. But as we’ll see, Mercedes has thrown everything it knows at this newcomer when it comes to performance and luxury-oriented tech. Our car’s gorgeous Patagonia Red paint is a £1,600 option, lifting the total to £149,075.

Tech highlights

Mercedes performance division AMG started with a clean sheet when it began development of this Mk7 SL. The result is an all-new roadster architecture, with a composite aluminium structure that shares nothing with its predecessor, or with any other Mercedes model. 

The new bodyshell’s torsional stiffness is up 18 per cent over the Mk6, while transverse rigidity is raised by 50 per cent over the AMG GT Roadster. Suspension is by a five-link arrangement all-round, with an all-new design up front. In a first for the SL, four-wheel drive is standard on every model. 

Modern performance cars have a vast array of electrical systems to keep their performance in check, and the SL is no different. The standard-fit adjustable dampers react to whichever of the six dynamic drive modes is selected. The same goes for the stability system, which adjusts its level of intervention, with more protection in Comfort than in Sport+, for example. It’ll even nip the inside brakes on corner entry to help the car turn.

Power comes from a 4.0-litre twin-turbocharged V8 with 469bhp and 700Nm of torque. The range- topping SL 63 gets a 577bhp version of the same V8, while the SL 43 has a 376bhp four-cylinder unit. Mercedes has confirmed that a performance hybrid variant will join the line-up later.

Safety: The SL hasn’t been subjected to Euro NCAP safety tests, but blind-spot assist, lane-keep assist and traffic-sign recognition are all standard.

On the road

Drive the SL and it reveals a character that feels slightly unsure of itself. It’s undoubtedly capable, but it doesn’t quite offer the comfort to nail the grand tourer brief, or have the sharpness to thrill.

Around town: With its low-slung driving position and noticeable width – 2.1 metres at the edge of the door mirrors – you’re very aware of the SL’s dimensions in tight spaces. Fortunately, the array of external cameras is excellent, so there’s little excuse to scuff that muscular bodywork. 

A & B-roads: The SL 55’s highlight is its stunning V8 engine. The 0-62mph time of 3.9 seconds delivers supercar pace, and power is always on hand thanks to the responsive nine-speed gearbox and vast reserves of torque. Traction from the standard four-wheel drive system is phenomenal, so even in wintry conditions it’s possible to exploit most of what the SL can offer. 

If there’s one snag about the way the car drives, it’s the ride. Even with the dampers in their softest setting, the body never fully relaxes on anything other than a perfectly smooth road, causing it to thump against the surface rather than flow with it. 

But find those better roads and there’s undoubtedly sophistication to the set-up, with body control kept well in check and a neutral chassis balance. However, stretch it beyond a certain point, and that 1,970kg mass begins to tell. Sharp, clear steering masks some of the weight, and it feels a little more agile than the BMW. 

Motorway: High-speed stability is one of the greatest strengths of the Mercedes, which means it’ll be just as reassuring to drive at 140mph on an autobahn as it is doing half as much on a UK motorway. Tyre roar is noticeable, though.

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Ownership

This is an AMG-developed sports car, but it hasn’t been compromised one bit when it comes to the sort of creature comforts you’d expect to find in a Mercedes with a six-figure price tag. 

Within the almost endless list of standard kit are features such as 21-inch alloy wheels, adaptive cruise control, keyless go, Nappa leather, wireless smartphone charging and augmented-reality satellite navigation. The cabin design feels sporty, with a snug driving position and a seat that can sit very low to the ground. Build quality is near- faultless, but the lack of physical buttons means that some functions are a little frustrating to control when you’re on the move.

Mercedes’s Digital Lights are also included. Each unit features more than a million micro reflectors, which allow the beam to be projected with “HD quality” precision while intelligently avoiding dazzle to other road users. They proved to be excellent during night-time driving, delivering a long, bright beam that shuffled around in real time, so that it kept its dazzling light away from other cars as they approached. 

As standard, the Mercedes comes with a three-year, unlimited-mileage warranty, which should help to cover any potential unexpected costs over the initial period of ownership. Used buyers (and very loyal first owners) will be reassured by the breakdown cover, which Mercedes provides free of charge for 30 years. 

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Heating: Keeping warm – or cool for that matter – is important in a cabriolet, and comfort  when the roof is down is boosted in the Mercedes by an Airscarf. This takes the form of small vents mounted inside the headrest that blow warm air on the occupant’s neck. Combined with the heated seats, it really works in cold conditions. 

The BMW comes with a similar system, which is just as effective. The SL also gets heating in both the seats and the steering wheel.

Practicality

There’s always a compromise to find between space and performance in a roadster like this, and Mercedes-AMG has certainly focused on the latter for the new SL. It’s a wonderfully luxurious place to sit up front, but elsewhere the newcomer is not so spacious. 

Rear Space: Mercedes calls the SL 55 a 2+2, but considering the tiny back seats usable at all is really optimistic. Even with an average-sized driver up front, there is no legroom whatsoever back there. Combined with the completely upright backrests, they’re really only good for extra luggage.

Boot: At 213 litres, the boot is fairly compact, yet Mercedes still reckons that two bags of golf clubs will fit inside. It’s not that the volume is tiny, but the opening is very small, so getting any sort of bulky luggage through it can be tricky.

What to buy?

Which engine and trim we’d choose…

Engines: The SL 55 is the middle of three choices. The SL 63 has an extra 109bhp and 100Nm for another £24,250.

Trim: The choice boils down to Premium Plus or Touring Plus. Kit levels are the same, but where the former gets black trim outside and carbon fibre in the cabin, the latter gets chrome highlights and aluminium interior pieces.

Options: Beyond the £2,500 Driver Assistance Pack, which adds self-steering, acceleration and braking for short bursts on a motorway, the extras comprise 12 paint colours and a black, red or grey roof.

Our choice: We’d opt for the SL 55 that we’ve driven here in Premium Plus spec. 

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BMW M850i

While the M850i is cheaper than the Mercedes on paper – prices start at £119,200 – there’s a much longer options list to bump that cost up. The car you see in these images has £15,000-worth of extras, bringing the total much closer to the Mercedes, at £134,200.

Tech highlights

Early 2022 saw the 8 Series receive a subtle range of mid-life updates. The changes include a reprofiled front bumper with revised air intakes, while the kidney grilles are now backlit for a more distinctive presence at night. The M850i also features the door-mirror design previously reserved for the M8 and other BMW M cars, while the rear diffuser is finished in dark grey, set against a range of new exterior paint finishes. 

Mechanically, the M850i is unchanged from before. Its 4.4-litre twin-turbo V8 isn’t just up on capacity compared with the SL 55, but it has a power and torque advantage, too. At 523bhp, it makes 54bhp more than the Mercedes, but that maximum isn’t spread over such a wide band – from 5,500-6,000rpm for the BMW compared with 5,000-6,500rpm for the AMG. However, the BMW’s 750Nm isn’t just 50Nm more than its rival, it’s also spread from 1,800rpm to 4,600rpm (it’s 2,000-4,500rpm for the SL). 

Below the M850i is the 840i, which offers six cylinders to the SL 43’s four, and the range tops out with the 616bhp M8 Competition.

Safety: As with the Mercedes, the BMW is available with an active lane-keeping aid, and it can even avoid or mitigate the risk of side-impact collisions. It’s part of the Driving Assistance Professional system, which is bundled into the £6,300 Technology Plus Pack. 

On the road

If the SL hasn’t quite found its place, the BMW is unashamedly a relaxing grand tourer. That isn’t to say that it’s all at sea through corners, though. 

Around town: Ride comfort is the most startling difference between these two. Even with a subtle sportiness to its gait, the 8 Series glides over surfaces that the Mercedes thumps into. The 8 Series is a large car in every dimension; at 4,851mm long, it’s 146mm bigger than the SL. It doesn’t feel unwieldy, though, thanks to fairly light, precise steering and a smooth-shifting gearbox.

A & B-roads: Those controls which make it quite relaxing around town certainly ensure there are no sharp edges on a country road. The brakes and steering feel like they have an extra layer of insulation compared with the crisp responses of the Mercedes; it’s not engaging for enthusiastic driving, but reassuring for the rest of the time. 

Still, there’s a neat balance to the handling that makes it very pleasing to drive at six or seven- tenths. On a straight piece of road, the shove from the engine is relentless. It doesn’t feel quite as dramatic as the Mercedes, but it’s barely any slower; you’ll not notice the BMW’s 0.2-second deficit in the 0-62mph run (at 4.1 seconds) in everyday driving. 

In its sportier modes, the BMW’s gearbox becomes a little more jerky, kicking you in the back as each upshift engages.

Motorway: If we had to drive to the south of France, we’d be hunting for a set of M850i keys before almost anything else. Performance, stability and comfort are all present in abundance, making it incredibly relaxing over long distances. The BMW will even reach 26 or 27mpg if you can keep it to a steady 70mph cruise.

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Ownership

As with the exterior, the M850i’s cabin also received some subtle updates for 2022. The most obvious is that all models gained a larger 12.3-inch touchscreen infotainment system (up from the older 10.25-inch set-up). The previously optional extended leather package, which covers the dashboard and door panels, is also now standard. 

More leather or not, the BMW’s interior is conservative in its layout, but there are absolutely no complaints when it comes to the quality. Fit and finish are impeccable, and even with the roof down, there’s not a hint of a squeak or rattle from any of the trim. The driving position is great, and there’s lots of adjustment for the seat and steering wheel. 

A useful number of physical switches means that basic ergonomic functions aren’t difficult to work out. Even the keys on the steering wheel are physical buttons rather than the haptic controls used on the SL’s rim.

Storage options here are better than in its rival, with a roomy smartphone cubby (complete with wireless charging) in the centre console, and there is even a pair of slim pockets in the doors. In most respects, the BMW’s warranty package is a match for Mercedes’, but buyers need to seek out their own breakdown cover because there’s only one year included as standard. 

Chassis: While the SL was designed to be a convertible from the outset, the 8 Series has been adapted from the design of the coupé. In order to compensate for the inevitable stiffness lost when chopping off the car’s roof, BMW has reinforced the drop-top’s chassis to compensate. 

Among the changes are a shear panel locating the mounting points of the rear suspension, and torsion struts to secure the double-wishbone front suspension. These changes, plus the added weight for the roof mechanism, mean the Convertible weighs 120kg more than the equivalent coupé. 

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What to buy?

Which engine and trim we’d choose…

Engines: The M850i is only available with the 4.4-litre twin-turbo V8. If you’re happy to lose two cylinders, then the 840i offers a £30,000 saving.

Trim: Since it’s a model in its own right, there is just the solitary trim level for the M850i, with a range of options available.

Options: Of those extras, we really like BMW’s Laserlights (£1,575), while the ventilated seats (which are part of the £1,575 Comfort Plus Pack) are a must-have for upholstery that might be left out in the sun.

Our choice: M850i xDrive with the Laserlights and Comfort Plus Pack options.

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Results

Which car comes out on top? 

Winner: BMW M850i

It doesn’t take long behind the wheel of the 8 Series to grasp that BMW set out to produce a continent- crushing grand tourer. It managed to fill its brief very well indeed. This is a comfortable four-seater that’s at home on a motorway or when gently cruising along with the roof down. 

Yet there’s more capability from the chassis than you might expect, and the engine hangs on to the coat tails of the Mercedes. Once the gulf in prices (especially with finance deals) is considered, then the BMW takes the win in this test.

Runner-up: Mercedes-AMG SL 55

Just as you’d hope from an AMG product, the SL 55 has bags of character, thanks to its wonderful engine and startling straight-line performance. The Mercedes is a great long-distance cruiser, and it handles fairly smartly. 

However, it’s neither comfortable enough to compete with the likes of the 8 Series, nor is it exciting enough to be considered a true sports car – especially on UK roads, where it’s just too firm to drive keenly. We can understand if you’ve fallen for its looks and noise, but there are flaws elsewhere.

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Rivals and other options

BMW wins our twin test, but what else is out there?

• Same class: Bentley Continental GTC
• Same Money: Porsche 911 C4 GTS Cabrio
• Used: Aston Martin DB11 Volante
• Used: Ferrari Portofino
• Coming soon: Mercedes-AMG GT

The great debate

What the Auto Express test team would do…

Sean Carson, associate editor: "Look at the SL 55’s specs, and it’s clear that Mercedes-AMG set its sights squarely on the Porsche 911 without considering the other competition. Unfortunately, the SL is too heavy and lacking in feedback to beat the Porsche, yet it barely offers any more refinement, either."

John McIlroy, deputy editor: "While the BMW is undoubtedly impressive, I can’t help but feel that the next step down in the 8 Series range offers 95 per cent of the performance while delivering near-identical levels of comfort and luxury. What’s more, the 840i starts from £88,160 – more than 30 grand less than the M850i."

Dean Gibson, senior test editor: "I’m looking forward to Maserati getting in on the GT act once again with the new GranCabrio. We’ve been very impressed with the Coupe version, so the soft-roof model might have what it takes to take on both BMW and Mercedes. And its turbocharged V6 engine will give it a unique personality."

Steve Fowler, editor-in-chief: "If there’s one thing that we Brits do brilliantly, it’s produce a great drop-top. Up your budget and the Bentley Continental GTC and Aston Martin DB11 Volante are both captivating, but the Jaguar F-Type R has bags of character and costs £11,135 less than the BMW. Production ends soon, so order one while you can."

Yousuf Ashraf, video content producer: "Let’s face it: the rear seats in these cars are never going to be used, so why not get something that’s more fun? The Chevrolet Corvette is even quicker than the Mercedes, sounds great and boasts a brilliant mid-engined chassis. It’s much cheaper, too."

What would you do? Let us know in the comments section below…

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Specs and prices