Having come from a B7 S4 to an E90 M3 the simplest way to compare is Land Shark (Audi) vs a Rocket (BMW). I did drive the RS4 a few times before I got the M3 and the RS4 handles like the S4 just with a little more HP behind it. The Audi's have more of a comfortable cruiser ride where the BMW is a bit more sporty.
I do love Audi interiors but the BMW is not too bad. So far I have to admit that the Audi Service is MUCH Superior to BMW's (My S4 was the last year of included free maintenance)
AWD does rock though, I kind of miss it, but I love my M3.