Ignore the fanboys who are merely justifying their own possessions as a corollary for their seld-esteem. Buy whichever one "feels" right to you. I loved my M3... but the C63 always lurked at the back of my mind. When they redesigned it and changed the suspension, interior, tranny, etc. I traded my M3 in for it after 1 drive and still do not regret it.
Current: 2012 C63 AMG PP | 2012 F25 X3
Past: 2008 E92 M3 | 2009 E90 335i | 2007 Z4MC | 2002 GTI 1.8T | 2000 Celica GT-S | 1999 328i | 1990 Corrado G60