As others said... the deal sounds a bit fishy.
Otherwise, do whatever feels right to you. Many here just don't want yet another M3 owner to trade in for a C63. I was one of them. My C63 coupe is a peformance package in Canada, has LSD and engine, brakes, etc.
Stock: I think the M3 is a bit livlier and probably a little quicker on a shorter track - on a longer, HP track the C63 will be better. C63 struggles with traction... don't know what AMG was thinking putting the same tire width as a C350 on the back. Motor sound - the C63 owns a stock M3 and many M3s with pipes. The car can be quiet, or an absolute hooligan. Love it. Will say, it gets more attention than the M3.
My FBO M3 was slower than my stock C63 AMG with the PP.
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