Audi RS6 Avant

Few cars at any price can match the four-wheel-drive Audi’s all-weather performance, plus the muscular cosmetic upgrades help it stand out. Further highlights include the first-rate cabin and high kit count. Yet it’s hobbled by the smaller boot here and a higher price, and while it’s blistering in a straight line, the rest of the driving experience isn’t as thrilling as the Merc’s.

Few companies can match Audi’s fast estate car tradition. Ever since the brand pulled the wraps off its Porsche-tuned RS2 in 1994, it has been leading the way with its lightning-quick load-luggers – and the latest RS6 is no exception.

Now in its third generation, the A6-based model actually has a much smaller engine than its predecessor, with the old 572bhp 5.0-litre V10 replaced by a 552bhp 4.0 V8. Yet despite the decrease in capacity and power, Audi claims the new RS6 is faster than ever.

The Audi’s styling certainly leaves passers-by in no doubt of its performance potential. With its bulging wheelarch extensions, large twin-exit exhausts and rear aerodynamic diffuser, the RS6 looks every inch a high-performance hero.

Our test car also featured the eye-wateringly expensive £4,250 Carbon styling pack that adds a revised front grille emblazoned with a garish quattro logo. Other head-turning additions include our car’s £1,900 21-inch alloys, which replace the standard 20-inch rims.

The racy theme continues inside, with heavily bolstered, high-backed front seats, a thick-rimmed, flat-bottomed steering wheel and a smattering of RS6 logos. Elsewhere, the cabin is standard A6, which means slick design and first-rate fit and finish. There are plenty of soft-touch materials, the dashboard is attractively styled and intuitively laid-out, and the low-slung driving position feels sportier than the high-set Mercedes’.

You also get plenty of standard kit, including sat-nav, leather trim, a powerful Bose stereo and four-zone climate control. There’s no shortage of space, with rear occupants getting plenty of room to lounge around – although the transmission tunnel gets in the way for passengers sitting in the middle seat. A deep glovebox, large door bins and an array of useful cubbies boost the Audi’s family-friendly feel.

In isolation the RS6’s 565-litre boot looks roomy, but it trails the Mercedes’ spacious luggage area by a significant 130 litres. Folding the Audi’s rear bench flat liberates a healthy 1,680 litres – although this is still 265 litres shy of the E63’s. However, there is the option of a useful £160 load-securing system, plus a standard-fit powered tailgate and a handy netted storage bin. Yet while the Audi is knocked for six when it comes to practicality, it strikes back at the track. Not only does the RS6 boast more power than the E63, it also benefits from four-wheel-drive traction and a launch-control system.

In wet conditions, our Avant blasted from 0-60mph in a mere 3.7 seconds, which was eight-tenths faster than the Mercedes. The Audi’s in-gear performance is equally impressive, and above 4,000rpm the RS6 accelerates even more ferociously than the AMG. Adding to the drama is the seven-speed twin-clutch transmission, which provides rapid-fire shifts via the steering wheel-mounted paddles, plus it delivers a crisp throttle blip on downchanges.

The RS6’s V8 may serve up blistering performance, but it sounds curiously muted alongside the vocal Mercedes. There’s no menacing burble at idle, while at high revs the 4.0-litre unit is muffled. A raucous sports exhaust delivers a more imposing soundtrack, yet it’s a £1,000 option. The Avant also feels a little subdued through a series of corners. There’s no shortage of grip and the upgraded air-suspension system provides rock-solid body control, but there’s not much feedback through the major controls.

Choosing the Drive Select system’s sportiest mode sharpens the throttle and helps reduce understeer by engaging a more aggressive setting on the electronically controlled Sports rear diff, but it also adds too much weighting to the steering. Make no mistake, the four-wheel-drive Audi is devastatingly quick on twisting roads, particularly in the wet, but it’s not as agile or engaging as the Mercedes.

Still, what the Audi lacks in driver involvement it makes up for with top-notch refinement. There’s virtually no wind noise on the motorway and in its comfort setting the air springs soak up bumps well – only deep potholes really ruffle the RS6.

At £76,985 the Avant is £1,450 more than the Mercedes, while it returned a thirsty 17.8mpg at the pumps. On the plus side, it has slightly stronger residuals than its rival. Neither of these rapid estates will be cheap to run, though. The question is whether the blisteringly quick Audi’s numb driving experience and smaller boot will hold it back in this encounter.