I've driven both the M3 and the RS5. To me they seem like totally different cars that almost belong in different categories. The M3 obviously has a great feel and is still the gold standard in the segment, but the RS5 felt like a refined spaceship for the road. It just goes wherever you point it, no drama about it. It seems like a car you would just take on the autobahn and GO, whereas the M3 seems like something you would toss around a mountain road instead, though I'm sure both would be fun in either situation.
In any case, I wouldn't mind adding any of these to my garage.