BMWbozo - so agree about the Acura TL - it looks nice (inside & out), the engine is great (but so "owned" by my S4 - as recently proven this weekend...along with [yet another Sti and a jaguar S R type - or some such...good weekend!
) - anyway - yeah - the TL is not a sports car - no way - feels like a souped up lexus - luxury way over sport. The clutch action was soft and very difficult to get used to - the torque steer was nasty (BTW someone had said something about torque steer in the S4 - come now...the only time I've ever really experienced such was in brutally hard acceleration on some rough road - and even then it was an awesome fel as the car was clawing foreward - anyway...) - and the body roll in the TL - forget about it...not at all the car for me - it felt much bigger then it was - and thats a (very) bad thing - IMO.
Regarding your dilema - a couple of things. First you need to drive each car and see which one fits you and your driving expectations. Secondly you need to think through how you'll be using the car - for instance do you need AWD (its awesome really - in most.many circumstances) - and will you be keeping car pretty stock or modding.
You may want to consider a used S4 - with the twin turbos. It is very easily modded to give impressive power (enough where I can take a new S4 without problem...). And with some more tweeks - suspension lowering and coilovers - that sort of thing - it handles and tracks like a champ - I have virtually no body roll (significantly less then the ZHP for instance). And the quattro - well what can I say - i can do some awesome things in it - wet or dry..and in the wet, in the snow - its just unbeatable - and I proven such many times...
All that being said the 330 ZHP is a fine fine auto that will provide pleasurable/exciting driving - with a much more compliant ride (S4 rides a bit harsh - but not too bad really - at least for me). It seems a tad more nimble with a tad better road feel then the Audi (but its very close) - and as someone has pointed out there is just a bit more backseat leg room. This was very noticible - even though it didnt seem like much. I drive with my seat quite a bit back. In the Audi there is barely room for my kids legs. My eldest - who is 11 - doesn't like it - even though he can fit. He was much happier with the room behind the seat in the BMer - so perhaps a consideration.
You can't go wrong with either vehical - why I am getting one of each perhaps...
....just need to determine whic is best for you...and the new S4 is a bit more expensive really...almost at the M3 level (and that would be a tough choice...again depending on what you want to do with it...where and when etc - I certainly like having no limitations [except maybe curb height etc] in the S4...but if thats [winter weather etc] not a concern) then maybe the BMer makes more sense - it may be just a tad more [easily] drivable...we'll see - I know I am going to enjoy it emensly....
Oh and one more thing - I think you have the 0-60 wrong for the new S4 - I think its more like 5 sec....believe me it is significantly faster then the ZHP - significantly - as is my twin turbo S4. To be honest - going form my S4 to the ZHP test drives the ZHP felt downright slow (sure some is the liniearity - but its really not in the same league - trust me. And 0-60 is far from teling the whole story. 0-120 or 1/4 mile times much better...even then...what I like in my S-4 is that at any point I can just hit it and if I don't quickly rein it in I'm in triple digits climbing fast - this is real world useable power. The ZHP has that too certainly - but at a bit more sedate level - or one must work the revs more it seems...