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It's while pausing for a break halfway up the side of an Italian mountain pass that the somewhat unlikely truth hits me: this is a twin test that seems to have only one car in it.

Up until now the scrap between the all-new S5 and the established force of the BMW 335i has been following the familiar script. One that, after years of Ingolstadt-versus-Munich ding-dongs, you doubtless know the gist of by now: Audi opens with four-wheel drive and a brilliant cabin, BMW counters with steering response and handling balance, with the contest then continuing over the subjects of performance, desirability and value for money before a winner is declared.

But just how different are these cars going to be? The moment of revealed truth comes as our contenders are parked nose-to-tail while photographer Hayden marshals his next shot. From my vantage point on the other side of the road I’m unable to see the front or back of either car, and it suddenly looks like we’ve brought two examples of the same model. In terms of proportions, stance, size and even the shape of the side windows, these cars look almost spookily alike.

Don't let the A4-ish front end fool you: this is far more than merely a numerically enhanced coupé version of the existing saloon and estate. Apart from shared powertrains from the rest of the range, the A5 is pretty much an all-new car, with major differences over Audi’s usual way of doing things.

Front suspension duties are now handled by a new twin-wishbone set-up, while locating the differential in front of the gearbox has allowed the whole front axle to be moved further forwards than you’d expect for a fundamentally front-driven Audi. It has also contributed to those 3-series-alike dimensions.

The steering rack has also left its traditional bulkhead position and is now located just in front of the axle line in a move that’s claimed to improve response and feedback. Oh, and the four-wheel-drive Quattro versions also come with the 40:60 front-rear torque distribution that we’ve been singing the praises of since it first appeared on the RS4. On paper, this is the sportiest non-RS badged car the company has produced so far.
Buyers will get to choose between the usual suspects of VW group powerplants. At the bottom of the range an all-new 1.8-litre TFSI petrol motor will be available from the autumn for those who can’t stretch to the heartland 3.2-litre FSI V6, while the familiar 2.7-litre and 3.0-litre V6 TDI diesels are also offered, the bigger motor tweaked up to deliver 237bhp and 369lb ft of torque. That equates to a 5.9sec 0-62mph time and an electronically limited 155mph top speed.

But it’s the range-topping S5 that brings us here. It’s the hottest version until the RS5 arrives next year and it uses the same 4.2-litre V8 petrol engine and six-speed manual gearbox that we’ve already seen in the S4. However, the motor has been slightly massaged here to deliver 350bhp against the S4’s 340bhp, delivered alongside 325lb ft of torque.

Even working against the porky 1630kg kerb weight, performance is still impressive: the 0-62mph dash is dispatched in a claimed 5.1sec and acceleration is barely diminished until a 155mph electronic limiter calls time. The 335i can’t match the S5 for on-paper firepower, but its twin-turbocharged straight six is still good for a potent 302bhp accompanied by 300lb ft of torque, while its official 5.5sec 0-62mph time is well inside the Audi’s ballpark.

And, suspense suspended, on the twisty mountain roads of northern Italy, the story then is pretty much the story now. The Audi’s powerplant has a wonderful soundtrack, its V8 burble becoming progressively harder-edged as the needle of the rev counter sweeps round.

It’s got a stronger top end, too, pulling all the way to the 7000rpm red line. But on real roads, and facing the challenges of passing real traffic, the Audi’s motor never feels as quick as the figures suggest. Low-rev lethargy and the slightly ponderous gearshift action mean that the BMW is the more effective overtaking weapon. Granted, the 335i’s twin-turbo motor can’t match the Audi’s on character or redline theatricals, although it still manages a pleasantly zingy soundtrack. But the combination of a small and a large turbo working sequentially has virtually eliminated lag and given the Beemer the sort of low-down torque that the Audi lacks.

But on the tight, Alpine passes north of Verona, the S5 comprehensively blitzes the 335i on A-to-B pace. We’re talking of a difference that would equate to several seconds a stage mile here.

The BMW has all the dynamic nudges and winks you’d expect to find. There’s nicely weighted steering, decent amounts of grip and an exploitable, neutral handling balance that allows the throttle pedal a reasonable influence on cornering line. But try to keep up with the S5 and it’s clear the BMW’s delicacy of touch isn’t winning you anything more than a growing gap.

The Audi feels nimble and more agile than the 3-series, turning into corners with far more enthusiasm and delivering near-perfect traction out of slower corners. Not that raw pace is everything, of course, and when it comes to finesse the Audi’s case wanes.

When Audi’s engineers talk about steering feel they’re clearly referring to something different than the rest of us. The all-new system suffers from the same over-light assistance that has marred most fast Audis for years. Nor does the rear-biased torque delivery do much for the handling balance, which still fades to steady understeer when the limit approaches. It’s devastatingly effective on the twisty stuff, but there’s still not much evidence of a dynamic soul.

So what about other coupé-buying criteria? Both cars have very corporate front and rear ends, with neither looking particularly great for it. From straight ahead the 3-series coupé looks virtually indistinguishable from a 3-series saloon, while the S5’s LED daytime running lights are the most exciting thing about its generic front end. And what’s with the Audi’s tail-lights? Design boss Walter de’Silva seems to only have one set – these are clearly the same as those he stuck on the 1999 Seat Toledo.

As predicted by the form book, the Audi has the better interior of the two thanks to its combination of quality materials and strong, sensible design with switchgear and instruments familiar from elsewhere in the range. Bits of the 3-Series’s interior don’t feel all that special in a £20k 318i, principally the ventilation controls, and so seem particularly outclassed in a car costing nearly double that.

The Audi is six inches longer than the BMW, but the 3-series is easily the more practical car, with usable rear seats and enough space for one six-footer to sit behind another in reasonable comfort. By contrast, any full-sizers relegated to the Audi will find their knees crushed and their heads pressed uncomfortably against the roof lining.

But it’s the question of money that brings the sting in this story’s tail. You’d be justified in expecting that two cars so similar in dimensions, performance and design would wear near-adjacent price tags, yet the Audi actually costs a whopping £6030 more than a 335i in SE trim, and £4155 more than the Beemer in M-Sport trim.

The S5 is a fine car. It’s better equipped than the BMW as standard and it’s guaranteed to be more exclusive than the 335i. But on the experience of driving the two cars back-to-back, it’s a price difference that can’t in all honesty be justified. It is the BMW that wins a close-fought contest here as a result.

The S5 is a good start but it’s hard not to see it as being a half-way house to the full-on RS5. The groundwork is certainly in place for that to be a truly memorable steer – and the fight between it and the forthcoming BMW M3 will be one of the epic battles of next year.

Edit: All credit to Autocar who published the review, more pics can be found on their website....