Can't go wrong with either. I've been doing some thinking and the interior on the C63 annoys me to holy hell. Another thing that annoys me is the fact that you have to pay Sirrious for live traffic. The M3 gets it for free using FM signals. It's great to have all that torque available.AMG has come a long way and the new coupe will probably best the M3 in every which way. Quality has certainly gone up. If Merc had cleaned up the dash a bit more, I think I would of jumped on one in a heartbeat.
P.S. I LOVE what they've done with the AMG Coupe. It has grown on me.