Leave the cars aside for a second; there could easily be an entire feature dedicated to the powertrains of the Audi S6 and Mercedes-AMG E53. The former uses electric compressor technology pioneered in the SQ7 to give its exhaust-driven turbo a helping hand in making diesel great again, alongside a 48v mild hybrid system. And the E53... also uses a compressor to assist a conventional turbo, albeit this time one bolted to a straight-six petrol, as well as a 48v onboard subsystem. Huh.
Mercedes-AMG goes further still, the 53-badged models claiming EQ Boost branding - for the 21hp/184lb ft starter alternator - as part of the manufacturer's hybridisation strategy. They're both very complex cars, then, especially so in the context of fondly remembered predecessors: once the Audi 'S' game seemed to be biggest engine fitted possible, with a 5.2-litre S6 once sold alongside a 4.2-litre S4, and the last sub-E63 E-Class was nothing more advanced than an E500. Very different prospects to what they used to be, though arguably the remit for both remains similar: discreet, imperious performance, peerless refinement, considerable luxury and the sort of desirability that comes with list prices in excess of £70,000.
On initial impressions both have delivered the sort of unerring and precise performance we've come to expect from the upper echelons of premium German brands. Both, too, answer the criticism that new cars are all a bit similar; straight-six turbo petrol and V6 diesel being two rather different ways of serving ostensibly the same purpose while targeting the same sort of customer.
As the fractionally newer car, and as more of a surprise development as a diesel, the S6 holds the greater intrigue initially. It's entirely as might be expected for a fast Audi: the discreet badges, the silver mirrors, the quad exhaust, the - how did you guess - beautifully appointed interior. Of course the difference comes when turning it on; there's still a noticeable diesel clatter outside at idle, though it manifests itself as nothing more than a muffled hum from the driver's seat.
Performance feels eminently familiar from many fast Audis of yore (note not all of them, before the protestations begin), dominated by enormous torque and a disarming ability to just keep accruing speed. For this remit, for delivering emphatic performance on demand, a diesel most certainly still has its place; while the EPC here doesn't feel as transformative as it did with the Q7-based equivalent - requiring another breath and a few more revs to really get motoring - the 3.0-litre is borderline remarkable in its response.
It's a soothing, smooth, beautifully calibrated powertrain, the eight-speed auto unfailingly accurate in its ratio choice - though 516lb ft papers over any mishap - and even the noise kind of pleasant, a gravelly and rough V6 rasp. Unless Dynamic mode is selected, that is, when the sound effects are somewhat overbearing.
Given the technology was present in 30-year-old cars like the Honda Prelude and R32 Skyline GT-R (bet you didn't expect those in this story) that four-wheel steer hasn't featured in Audi S cars before now is bizarre. Typically (in)famous for their nose-heavy balance and inert, stodgy handling, a steering rear axle would surely have worked wonders dynamically for the older models. Still, it's here, optionally, on this S6, so let's be grateful. Because, wouldn't you know, it's pretty transformative.
There's an alertness and agility to the way this S6 changes direction that confounds expectation - or prejudice, more accurately - the large diesel estate slicing through bends like a car hundreds of kilos lighter. The (also optional) air suspension keeps a reasonable check on proceedings, too, and even permits a more-than-tolerable ride quality in Dynamic mode, meaning ground can be covered at speed that would be hard to tally with the sober appearance.
What a shame, then, that there's precious little joy to derive from the experience - again largely conforming to type. The steerable back axle is a more than worthwhile addition, but the odd steering weights at the rim don't make it easy to judge the extra agility. The braking power is immense, though with a pedal that's hard to modulate pressure on. And the more attention is paid to them, the more it seems that comfort is a suspension setting that's a little too languid, and dynamic one a bit too abrupt. 'Auto' becomes the Goldilocks almost-right setting for that, and for all other parameters really; the not-quite-perfect compromise more acceptable than too much of one or the other.
But that shouldn't matter, really - should it? To a large extent, no, because this S6, despite its fuel source, delivers everything that we've come to expect (and buyers desire, don't forget) from this type of Audi. And it's a pretty alluring package: fast, able, stylish, just sufficiently opulent to not feel OTT. The S6 is the kind of car a customer should beg to collect from the factory, because a drive home from Germany would display its manifest talents - the speed, the refinement, the sense of consummate ease - just perfectly.
The issue lies in the red corner, because the Mercedes proves it is possible to deliver a sumptuous luxury wagon with genuine driver reward. That begins with the engine, the electrified straight-six being a work of genius. There are all the benefits of hybridisation, in terms of step-off spontaneity and instant response - with the turbo torque advantages and a sweet straight-six following on from those impressive support acts. Even with a just-less-than-perfect gearbox, the 53's engine is seemingly never caught out.
Consequently, a torque deficit that looks considerable on paper never materialises on the road, since everything is so immediate and the band so large, with the E53 as capable of low-rev hauling as it is 6,500rpm screaming. With large displacement engines no longer feasible, clever (and fiendishly complex) alternatives are going to have to be found, and this 53 arrangement is probably the most convincing yet: it simply behaves like a much larger straight-six, with no hint of the turbocharging or electrification witchcraft also at play. Just as relevantly here, it's a more exciting, more potent engine than the Audi's, and not staggeringly less efficient - 22mpg played 28 on our shoot.
To drive, the Mercedes feels more of an AMG product than the S6 does anything from the Audi Sport catalogue, which is a more important difference than it sounds. It isn't a feral, deranged, six-cylinder E63 kind of car - even if the toughness of its low-speed ride feels familiar - instead the E53 simply delivers an impression of cohesiveness and engagement that eludes the awkwardly disjointed Audi. Steering response is more consistent (yet the car doesn't seem to give up much in agility), brake feel is more natural, the response from the car to your inputs having greater predictability. The suspicion is of greater outright ability, too, the E53 authoritatively shrugging off lumps and bumps that might see the S6 fluster.
Truthfully, these cars require a more extensive test than our afternoon with them could provide. Unearthing their respective talents feels like the work of weeks and months. Half a day tells you they are faster than most other things on the road, less imposing than an SUV, as capacious as a small van and no less cosseting than a bonafide limo.
Crucially for both manufacturers, there's clearly space - both in terms of outright performance and philosophy - above the E53 and S6 for their range-toppers. They now sit with discrete (and desirable) positions in their ranges, which must have been a harder trick to pull off when both S6 and RS6 were powered by V10s (or V8s), and similarly for AMG Mercs. They make convincing cases for themselves on their own merits, and resist the accusation that they are a poor man's flagship.
The superior contender isn't hard to choose, however. While the S6 is an extremely likeable car in isolation, one that feels to benefit significantly from new technology while never being defined or overwhelmed by it, driving it doesn't feel as memorable as it did in the SQ7. Perhaps that moment for diesel can only happen once. Regardless, the resounding impression here is of an A6 made faster and sharper, rather than a vehicle of genuine sporting prowess. Perhaps a back-to-back drive with a 50 TDI would clarify the situation, but the fact remains that an experience befitting the £70k asking price never quite materialises.
The Mercedes, on the other hand, is additional proof of the impeccable run of form AMG is currently on. It comes across as both immediately desirable and yet also lastingly appealing, the combination of E-Class opulence with AMG potency familiar in its charm - only now further embellished by the practical benefits of hybrid and compressor technology. A ride with some rough edges and the occasional reluctance of its gearbox deny the E53 a perfect score, but the case it makes is compelling. If this is the near future, there won't be any complaints from us.
SPECIFICATION - MERCEDES-AMG E53 4MATIC+ ESTATE
Engine: 2,999cc 6-cyl with exhaust driven turbo, electrically driven turbo and mild hybrid motor/generator
Transmission: 9-speed automatic, all-wheel drive
Power (hp): 435@6,100rpm
Torque (lb ft): 383@1,800-5,800rpm
Top speed: 155mph (limited)
Weight: 2,020kg (EU, including driver)
MPG: 31.7 (NEDC combined)
Price: £65,790 (£72,455 as tested, comprising Premium Plus package with Panoramic roof, Burmester surround sound and Keyless-Go £2,595; Hyacinth Red paint £685; Driving Assistance Plus package £1,695; Comfort Package with Air Balance fragrance dispenser and Energising Comfort Control £395; 20-inch AMG wheels £1,295)
SPECIFICATION - AUDI S6
Engine: 2,967cc, V6 diesel (with electric compressor)
Transmission: 8-speed Tiptronic auto, four-wheel drive
Power (hp): 349@3,850rpm
Torque (lb ft): 516@2,500-3,100rpm
0-62mph: 5.1 seconds
Top speed: 155mph
Weight: 2,020 (DIN unladen)
MPG: 35.3 (WLTP, 20-inch wheels)
Price: £62,745 (£77,980 as tested, comprising Tango Red metallic paint for £685, 21-inch '5-twin-arm' wheels for £1,800, Sports differential for £1,550, Tour pack for £1,950, City assist pack for £1,375, Parking assistance pack for £700, Matrix LED headlights for £600, Extended LED interior light pack for £275, Audi Music Interface, rear for £175, Acoustic Glazing, side windows for £525, Adaptive air suspension for £2,050, Electric steering wheel adjustment for £425, Panoramic glass sunroof for £1,950, Extended leather package for £375 and Bang & Olufsen sound system for £800)