Quattro is flat out unbeatable in bad weather situations, especially snow. If you're dailying the car, and do a lot of commuting in snow, it would be hard not to want an RS4 over the M.
The carbon build up is definitely an issue. I wouldn't care about suspension cause you should upgrade it all anyway. The 4.2FSI is a great engine, but frankly I find it underpowered. I was pretty pissed when Audi put a NA engine in an RS badged car. I think history may soon suggest that the B7 will be the worst RS badged car Audi has built.
Personally I don't normally like the aesthetics of B7 Audis, but the RS4 with those massive fenders pull it off. Kind of like the RS6 pulled off the alien jellybean thing the C5 A6 had.
having spent a bit of time behind the wheel of an E92 M3 lately, I agree with what others have said, dry handling there is no comparison. The M3 is just an awesome car. For the same money I'd do the M3.
(and keep a stage 3 B5 S4 in the garage)