S4, hands down. I have a B6S4 and love it. It is a beast. The engine is one of the best ever and the cams are chain driven (in the oil sump). It also has 4 door practicallity. The Recaro seats are a masterpiece. And, you can (and IMO must) get the 6 spd manual (which IMO has ruled out the R32 for me, period). You are comparing a $55k car to a $33k car. The reliability and fuel consumption will be about the same. I would try to find a certified one. And the 05's are better than the early 04s. Lurk and read in the Audiworld.com formum for these to get an idea what's going on with these.
http://forums.audiworld.com/s4b6/. The 335 is truly a masterpiece, but not if you live where there is snow, plus you will not find one priced like the B6