Id say the m3 all day long, ive owned an e46 m3 and currently own a b7 audi rs4, in terms of performance the audi is only a bit quicker so i'd imagine an s4 to feel a bit slow compared to the m3 and a bit boring to drive. M3's are great on track too i did a trackday in mine and it was awesome for a road car, they really feel like there made for it. As said before buy one thats had the boot floor checked or repaired as it could be a headache for cars older than 10 years which will be almost all of them now. Lots of great info on the m3 cutters forum abour common problems and expenses