The keen among you will have noticed an obvious difference between these two. One is red, and one is blue. Oh, yes, and one has a roof that’ll go down – the other doesn’t. Logic would dictate that SL versus M8 Convertible would be the test to conduct, but experience of the latter suggests otherwise; AMG’s ground-up hot rod against BMW’s biggest boulevardier wouldn’t even have been close. The inevitable 911 duel will probably happen in time, but this heavyweight head-to-head had plenty going for it as well. This is BMW M against Mercedes-AMG on ground not covered for a long time: a pair of twin-turbo V8s loaded to the gunwales with luxury, technology, and horsepower. M8 and SL55 are closely aligned enough to be separated by just £10k before options, which isn’t much at all in this sphere. They ought to be cars as appealing for first thing on a Sunday morning as they are a week-long tour of Europe – so which does it better?

There’s only one winner for showroom appeal. The BMW is the same M8, really, as it was at launch in 2019, large and plush and nice – but very recognisably BMW. Its sheer size ensures presence and it remains a handsome beast, certainly, though the 8 Series was never the most outlandish car, and with time has of course comes familiarity. The SL combines both the novelty value of being the new kid on the block with some smart design as well, neatly straddling the attitude of the old GT Roadster with the suaveness expected of a flagship. It’s a taut looking two tonnes in its big Michelins, clearly cut from the AMG cloth albeit with Mercedes influence as well. To these eyes, it’s really attractive. And easily the best SL in years. 

That same personal caveat applies inside, however; plenty like what Mercedes is doing with design right now, but just as many seemingly do not. That huge screen and button busy wheel are liable to discombobulate some (NNC admitted to being briefly flummoxed in the ’63), although it’s definitely attention-grabbing. By and large, it works nicely as well, even if some processes (such as the roof) are needlessly fiddly. Mercedes has taken some flak elsewhere on perceived quality, and truthfully it isn’t where it should be for a £150k Mercedes – certainly not with display glitches factored in. Assuming everything works in the showroom, it won’t prevent most buyers from being impressed – there’s a feelgood vibe even with some questionable trim choices – although perhaps a big portrait screen isn’t as novel or as captivating as Mercedes thinks it is. Having said that, the M8 makes feel a bit like you’re in a 4 Series with some nicer leather. Which is a fair reminder that a really smart driving environment at £50k doesn’t quite cut it at £100,000 more. They both have familial links, then – the SL’s just seem to fit better.

If there are mixed messages from the SL as a static object, the AMG-ness is plain enough on the road. It feels taut, precise, and focused in a manner that anyone familiar with AMG coupes will recognise – and which will seem alien to those expecting just another SL. The R232 feels like an AMG first and an SL second, which is an important role reversal from the old days, a sense of rigour applied to the important stuff like chassis rigidity, damping, assists and so on. Rather than just a honking V8 wedged into a big cab.

What a V8, though. Even in its most modest state of tune and years after its introduction, the 4.0-litre M177 unit continues to charm with its response, sound, and performance. It does everything you’d want of a large AMG V8, picking up from very few revs and never happier than plundering its generous reserves of mid-range muscle – yet also willing to rip around to 7,000rpm as well. The nine-speed auto remains a decent match. And nobody needs reminding how joyful a big V8 can be with the roof down, huge bonnet reaching out ahead and bum down on the rear wheels. A new SL it may be, but it can do old-school satisfaction as well.

Sports car feel courses through the SL, which is some achievement for a two-tonne Mercedes. That Comfort is as tense and alert as it is says much about AMG chosen level of intent, making Sport probably the best compromise of ride comfort and control. It feels remarkably together for a roofless car, and only builds on that with monster grip, supreme traction, and swift turn-in, the latter aided by four-wheel steer that could perhaps do with a bit more sensation. Clearly, this new 55’s ability is on another plane compared to SLs that have come before; it’s obviously not an E63 cabrio, but that same grit and tenacity is here. It might still accommodate golf clubs (and tiny people, at a push), but the SL means business.

It’s amazing how relaxed the M8 seems by comparison. Oddly (and ironically) enough, the initial impression is actually of an old-school AMG, where the experience is dominated by an outrageous amount of horsepower. The SL is fast; the BMW is extraordinarily rapid, packing another 150hp from its slightly larger V8 into a car that weighs and costs about the same. The torque advantage is less than 50 points, so the M8 only feels a little stronger than the AMG once clear of its low-rev stupor – working towards its 6,000rpm power peak and 7,200rpm cut-out, however, the BMW is rampant, hilariously and wantonly fast even in this company. The gearbox might not be as keen and the noise less inspiring, but there’s no arguing with the BMW’s monstrous performance. Or its entertaining aftershock.

Especially when you consider the M8’s surprisingly relaxed attitude. It might be called Competition and it might offer all sorts of configurability, but this feels a big bruiser at heart – less focused than something like an M4, and more liveable after a recent facelift softened it off a bit. Comparing this BMW with the SL feels like the M5 v Panamera test we did a while ago. The less powerful car with roots in Stuttgart is probably just as quick across the ground, with its battery of chassis tech, prodigious grip and seemingly limitless composure; the BMW, with less rubber on the road, a more rear-biased four-wheel drive system and just one steering axle, is more willing to play the traditional entertainer. Here, in the AMG, you’re seeking the next opportunity to witness it cling to the road or turn in at an unfathomable speed; all you want in the BMW is to feel it squat and squirm from the next bend as that V8 fury is unleashed one more time.

We could (and owners should) spend many days configuring the ideal chassis set-up for both, so it’s hard to draw definitive conclusions from a day in their company. That said, it’s notable (and surprising) how the BMW, despite the best effort of its V8, can never quite match the SL’s sense of overall urgency; whether it’s throttle response, wheel control or steering speed, when you want to amp things up the AMG is the more obliging companion. The flipside is a more cossetting M8 when required, an advantage it builds upon with greater rolling refinement. The SL is always noticeably firmer, and suffers from a lot of wind noise, which is irritating when you’ve stopped doing a very good impression of Stirling Moss.

It’s hardly like the M8 is bad to drive fast, – this is essentially a two-door M5, after all. So while it stops short of seeming truly exhilarating, it sure can hustle, managing its mass well, and shedding speed incredibly quickly with the optional ceramic brakes. But your smile will probably feel broader in the SL, at least in this arena, as it does a more convincing sports car impression.

Well, up to a point. Just when it’s hoped the SL might move from really sorted fast car to AMG legend, it doesn’t quite commit to making the final leap. Even the geniuses of Affalterbach can’t make a 2,000kg cabrio perform miracles; the odd moment of shudder, a less than brilliant brake pedal (plus a seat belt that won’t stop strangling when you push them) and steering too light on feedback robs it of an exemplary score. Its performance bodes very well for the upcoming GT coupe, but the inherent compromises required of an SL come to the fore eventually.

It leaves a quandary in its wake. AMG has attempted so many things with the first SL handed to it from day one that it risks failing to properly beguile you on any one score. It’s not as refined or as cushy as some might expect an SL to be, and nor is it (quite) as good to drive as the best back catalogue AMGs. That doesn’t mean it’s unlikeable or hard to recommend – far from it – but you should know you’re not precisely getting one thing or the other. A softer, nicer trimmed and better-insulated car might’ve resulted in a more traditional SL; a more single-minded AMG would do a better job of blowing your socks off. Instead, the result is part AMG sports car, part V8 hot rod, part tech showcase, part luxury car – which is fine as far as it goes, and probably as its maker intended, yet also sometimes as compromised as it sounds. 

Tellingly, never has the M8, with its pair of brake pedal weights, three steering modes and whole heap of other tech besides, felt such an uncomplicated, endearing prospect. It goes fast, looks good, and fundamentally drives well. It’s not the best M car out there by several notches, but it only takes a romp through a few corners (or a saunter down the motorway) to see what’s what. The M8 does exactly what might be expected of a big, powerful BMW, and does it very well – there’s something comforting about that, not to mention likeable.

In contrast, the latest SL is all about redefining what we can expect from the badge. That’s no bad thing: it means the car has developed a willingness to be driven hard, and possesses the chassis and engine to underwrite its new AMG-forged image. But Mercedes has fallen short of the high expectations set for it elsewhere, and any buyer accustomed to the six-figure sporting GT market is likely to notice the deficit in comfort and overall finish compared with some rivals. As an open-top driver’s car, it runs the M8 sufficiently close to nick this test by the PH rules of engagement, though it would probably not fare so well against the cheaper 911 GTS by the same measure – or against (the admittedly much pricier) Bentley GTC S, were luxuriousness and refinement the benchmark. 

Mercedes-AMG will gleefully accept the M8’s scalp, regardless – and it should. Especially if it were inclined to concede that the 55 is merely the stepping-off point for this new generation of SL. The more sophisticated and much more powerful SL63 has already shown itself to be worthy of a longer look in time, and there is still the prospect of the four-cylinder SL43 to come. Sniff at the latter if you like, but on this experience, the thought of a lighter, cheaper-to-buy and exclusively rear-drive SL is an intriguing one. Because there’s a great car in there somewhere. Don’t bet against AMG eventually locating it. 

Engine: 3,982cc, V8, twin-turbo
Transmission: 9-speed auto, four-wheel drive
Power (hp): 476@5,500-6,500rpm
Torque (lb ft): 516@2,000-4,500rpm
0-62mph: 3.9 seconds
Top speed: 183mph
Weight: 1,970kg
MPG: 21.9 (WLTP)
CO2: 292g/km (WLTP)
Price: £147,475

Engine: 4,395cc, V8, twin-turbo
Transmission: 8-speed auto, four-wheel drive
Power (hp): 625@6,000rpm
Torque (lb ft): 553@1,800-5,860rpm
0-62mph: 3.2 seconds
Top speed: 190mph (with M Driver’s Pack)
Weight: 1,900kg (DIN)
MPG: 25.0 (WLTP)
CO2: 259g/km (WLTP)
Price: £137,850