You honestly can NOT go wrong with either car. Both will make you go "from 6 to Midnight" lol. I'm fortunate to have an M3. I would be fortunate to have a C63. Respect isn't lost for one because I own the other.
Perfect combo for me would be either a E90 M3 Sedan for a DD and then a C63 Black Series for the weekend OR a C63 AMG Sedan for a DD and a M3 GTS for the weekend.