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The Audi S8 and Mercedes-Benz S63 AMG are limousines with a difference – where the rear seat takes a back seat.

These are the fastest, most powerful versions of the A8 and S-Class in Australia, where those buyers typically behind the wheel of their own successful company kick chauffeurs out of the driving seat.

Those annual dividends will need to have more zeroes on them if you want to stretch to the longest Benz passenger car over the longest Audi passenger car.

With no RS8 (or BMW M7 for that matter) in existence as a more natural rival, the Audi S8 requires a cheque for $279,900 where the Mercedes S63 AMG asks for a banker’s draft of $385,000.

Bigger money doesn’t guarantee universal big-noting in the boardroom, however – at least in Australia.

The S8 is quicker to zip from standstill to 100km/h – taking 4.1 seconds to the S63’s 4.4sec.

The Audi’s advantage comes in its standard all-wheel-drive system. The S63 is also available with AWD that takes it to an S8-pipping 4.0 seconds in the 0-100km/h, though only in left-hand-drive markets.

(A trip to the Western Sydney International Dragway – yes, we really took these cars there – proved the Audi’s off-the-mark superiority, though the S63’s traction is outstanding – watch the video here.)

So in Australia the Super S-Class must put all its mumbo through only the rear wheels. And what mumbo it is.

The Mercedes-Benz S63 AMG now produces 430kW of power and 900Nm of torque from its tweaked carry-over 5.5-litre twin-turbocharged V8.

Great sedans clearly think alike, because the Audi S8 also opts for a pair of turbos and eight cylinders in a vee configuration.

But with a 4.0-litre engine that’s about 40 per cent smaller than the Benz V8, there’s a commensurate ‘shortage’ of torque: 650Nm.

However, the Audi does spread its maximum torque across a wider rev range – 1700-5500rpm where the Benz’s 900Nm is delivered from 2250-3750rpm.

Interestingly, the Audi S8 and Mercedes-Benz S63 AMG were evenly matched in our 80-120km/h test – where the number of wheels being powered is less of an issue.

You can sense the vast reserves of power beneath your right foot in both cars even on the lightest of throttles.

At lower speeds, the Mercedes’ throttle offers the crisper, smoother response, though plant a heavier right foot and there’s an initial delay before a mighty surge of forward momentum kicks in.

The Audi is the car that accelerates more seamlessly at higher speeds and makes you question whether it really is turbocharged.

It was the consistent winner in our rolling response comparison, though the fact the S63 was just a poofteenth behind in the fastest times recorded – 3.65sec versus 3.70sec – reveals it’s the limo that would reach its top speed first (300km/h for our test Merc that featured the AMG Driver’s Package that overrides the 250km/h limiter).

If revs equal power, though, the S8 is the hungrier of the two V8s to reach it despite being nearly 50 kilowatts behind (382kW) – encouraging you not to rush into the higher gears so hastily.

Acceleration is also accompanied by a wonderfully addictive staccato soundtrack.

The S63 AMG goes about its business in a far more muted manner. Selecting Sport or Manual for the transmission opens a flap in the AMG exhaust system to deepen the V8’s growl more readily, though it’s still a more restrained sound than the same engine (in detuned form) makes in the E63 AMG. It’s deliberate, of course – for a different target market.

Regardless, these two limos are remarkably rapid machines that simply crush long distances – and make overtaking a far quicker and less complex affair than a corporate takeover.

Both transmissions – an eight-speed auto in the Audi and a seven-speed auto in the Mercedes – excel at smooth, timely shifting when upping the pace. Paddleshift levers are standard, too, for drivers wanting to assume total control of engine speed.

Fuel consumption for these kinds of cars can only have relevance for thinking about the planet rather than the wallet, and it’s the Audi S8 that is kinder to the environment with an official figure of 9.6 litres per 100km against the S63’s 10.2L/100km.

If you’re in a hurry to get to the country mansion the S8 will get you there quicker if tight corners, or wet roads, are part of the route.

Here the S8 feels lighter on its feet than the S63 despite being the slightly heavier of the two-tonne sedans, and its all-wheel drive comes to the fore.

Tip both into the same second-gear left-hander and the Audi allows you to jump on the throttle earlier to let the extra traction do its magic and fire the S8 out in a straight line.

The S63 is a game of patience in contrast despite equally strong tyre grip, where the nose needs to be further around the bend before the driver applies the throttle more progressively. Too eager and the orange stability control warning light on the S63’s dash starts to pulse.

Yet the Mercedes still corners with an impressive flatness, and a trip to the racetrack with it (yes, we really did that, too) proved it doesn’t embarrass the AMG badge.

And it’s the Benz that provides the vastly better steering and body control.

Both steering systems are on the light side, in keeping with their limo remit, though while the Audi’s helm is vague and numb, the Merc’s wheel, apart from being slightly over-servoed on initial turn, is smoother and more accurate.

Both limos feature suspension that can be adjusted by the press of a button (S63 AMG) or twist and push of a dial (S8).

The Audi offers Comfort, Auto and Dynamic modes, though none enables the S8 to maintain the same composure as the S63 over typical Australian country roads.

The firmer Dynamic setting is a must if you don’t want the S8 to come close to bottoming out through big dips or allow too much vertical body movement over big bumps.

Switch to the Benz over the same stretch of road and it sits flatter whether you’re in the default suspension mode or the Sport mode that further stiffens the AMG sports suspension. You do hear more tyre noise than in the Audi, though.

But back to the real world where these cars will be driven for 95 per cent of their existence.

And whether you’re cruising along the freeway or commuting to the CBD, the Mercedes-Benz maintains its ride quality advantage.

Where the Audi S8’s reactions to surfaces constantly informs the driver that they’re in a sporty sedan (rolling on 21-inch wheels to the Benz’s 20s), the S63 AMG plays the role of guardian angel in protecting its occupants from the evil world of potholed, patchy bitumen.

The three-pointed-star’s Magic Body Control system, which scans the road ahead and then relays instructions to the dampers to make millisecond adjustments for upcoming bumps, is a ‘gimme’ rather than a gimmick. (Sport mode turns the system off, by the way.)

The only chink in the S63’s suspension armour is sharp joins that the front end can’t quite filter out.

Seats are an essential part of comfort, and here the Mercedes is a mile ahead with its armchair-like pews in which you could spend a whole day thanks their just-right blend of chunky cushioning and supportive security.

The S8’s seats are merely good in comparison – thinner, firmer and a touch underdone on the cushion length.

This is indicative of the approach to the interiors by both car makers.

There’s a significantly sporty emphasis to the Audi’s cabin – different in philosophical approach to the Benz – with large sections of carbonfibre applied throughout to mix with the range of quality materials found in regular A8s.

That expensive weave is available as an option for the S63’s interior, though in standard form the AMG factor is relatively subtle. The logo of Affalterbach – the home of AMG – is embossed into the console bin lid, there are AMG badges on the sills as well as the beautiful two-spoke steering wheel that features Alcantara handgrips.

Apart from that, it wouldn’t be immediately obvious to an S500 owner that they had stepped into a more expensive S-Class.

The opulence of that new Mercedes-Benz S-Class interior – which we cover in more detail in our S500L review – is already untouchable in its class, however.

There’s a level of craftsmanship, materials quality and exquisite detailing that looks and feels a cut above the Audi. The dual TFT screens across the dash also contribute to the sensation of sitting in a more technologically advanced car (which the Benz is).

The S8 wins some points back in the interface battle, with its MMI system proving to be generally more intuitive than the S63’s Comand set-up – even allowing for the fact the Audi features an array of buttons where the Mercedes opts for minimalism and multiple layers or rotations of menu screens.

Places swap for high-end audio. While our S8 featured a $14,500 Bang & Olufsen system, it isn’t especially crisp and its bass is a bit muddy.

The standard Burmeister system in the Benz is the clear choice for audiophiles, with a beautiful bass and extraordinary clarity that encourages you to turn the volume up higher than you would in the Audi.

And those rear seats? We won’t go into detail here – buyers of either model simply need to know none of their passengers would complain about the space or comfort available. Our S8 came with an optional rear DVD entertainment with own control panel, though the S63 is standard with electrically adjustable outboard rear seats.

For there to be a choice in the first place, of course, money needs to be (almost) no object. Even at this price point, the $100K-plus gap can’t be dismissed too quickly.

And for under $300,000, the Audi S8 has a portfolio of attractive points – notably its terrific refinement and that wonderful drivetrain.

But if the company profit and loss sheet has plenty of black ink on it, the Mercedes-Benz S63 AMG proves its extra value in a number of ways – even if it’s best to approach it as the swiftest S-Class rather than the ultimate AMG sedan.

Where the S8 feels like a bigger version of S4, the S63 feels more special than just an upsized C63.

You sit in an environment that feels more expensive than the S8’s, and a greater selection of technology, a more cosseting ride, lovely steering and equally alluring twin-turbo V8 make the Mercedes-Benz S63 AMG the richer sporting limo experience.

Click the Photos tab above for more images of the Audi S8 and Mercedes-Benz S63 AMG.

Photography: Mitchell Oke, Jez Spinks, supplied