If you're only talking about straight line performance, then the V will own. Even on the twisties, if driven correctly the V will show its greatness. Now, throw a set of the countless coilovers offered for the Audi's on the S4 and add a matched set of sways and slightly larger tires...that's a different animal. Properly modded, and by properly I'm referring to a car's weaknesses, the Audi can be a phenomenal car for the track or street. I won't even bring up the mis-match we'd have if there was any rain or snow involved...
All in all, its a great car...almost had my money, but doesn't have the "rip-yer-balls-off" charisma of the V. And to make that S4 the track animal it can be...would take a nice pile of cash.
-ace