If you ask me, C63 all the way. That is, if you are going to be just driving it on the street, the C63 seems to be the best choice. All of that torque makes it a great daily driver like the 335i, but with seriously more power. Seems like it would be a handful and extremely fun to drive, as Clarkson described it as, "An axe murderer with headlights."
The exterior of each car appeals to me, but when comapring the C63 and M3 sedan, my vote goes toward the C63 for the more dramatic styling. Also, with the black birdseye maple trim in the Mercedes, the interior (with the non-dual hump dash) of the C63 appeals to me more as well.
The M3 would be the better street/track hybrid car because of the handling and lower overall weight. However, I doubt the M3's ability as strictly a track car when leaving it with its stock configuration. Upgraded brakes seem to be a must, according to recent reviews.