May as well throw my two bobs worth in here.
First, let me start by saying if I had the cash I would take an RS4 any day of the week.
A while back I had an RS4 on overnight loan. I've also driven an R8 and an RS6.
As far as out and out acceleration is concerned, the RS6 is the only one of the three to not leave me slightly disappointed. I guess 572bhp has a bit to do with that, haha.
Back to the RS4, after my first decent blat I must admit my first thought was, for AU$180K one would expect a lot more go for the dough. I did, indeed think, that AU$70K for an S3 with $10K in mods would leave one with a car that offered just as many thrills as the RS4, and $110K change, haha.
Over time, though, the RS4 won my heart in a big, big way. As noted power delivery is so linear and the V8 just loves to rev. IMO, it is because the car is so competent that it might feel slow. But it's not. And, of course, Audi Avants are officially the coolest cars on the planet. That, and the 4.2 V8 is one of the sweetest sounds ever heard.
My drives of the R8 and the RS6 were over much shorter timeframes (about an hour each), and while the R8 was a blast, I reckon the V10 will be the one to get. And a manual box as well, that R tronic thing was a bit odd.
The RS6 is simply out of this world so far as pure grunt is concerned. And the V10 sounds sublime (see below). After driving the RS6 I did say the RS4 was dead to me, but the RS4 Avants just look so damn cool, that I would easily go back on my word if the opportunity ever presented.
Ulitmately, the S3, RS4 et al are all damn fine cars and if we're lucky enough to own any of these we should all count our blessings.