I think what the m3 has going for it is it's handling.
Other than that... Damn the c63 is awesome. I'm actually looking at some c63 next weekend. It is not as sporty as the m3 but it has a nice motor and sound which is important to me. I don't track my m3 do I'll never experience it's true potential anyway. For a driver like myself, c63 is perfect.
The only thing is that I find the c63 to not hold its value as well as the m3.
Isn't this beautiful?: