I've had both and C63 wins it for me. The car is not without faults. Some cannot be remedied (glass roof), others can... to an extent (rear tires).
My M3 was FBO + tune. Just for the inevitable track comment, yes i do track and have tracked both my M3 and my Z4M (rear exhaust, intake / scoop only) before (in addition to other cars).
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