The latest R&T has a comparison between the C4S and RS4. I had also considered the new RS4 when I ordered my C4S as I had really liked the quality of my old Audi TT. The RS4 is really fast,...almost identical times in acceleration vs the C4S - so, if you find yourself in a race, go for the 155-mph governed speed against the Audi!
Enjoy!
thx both very good cars, and i've see nthe RS4s driven as safety cars at le mans, they're pretty darn fast! and they sound good as well, but stilll i'd have the C4S