I've been in exactly the same boat for a few weeks now, torn between the two. I've decided on the C63 over the M3, mainly because I don't do any track days anymore so would be road use only, plus the noise of the merc is unreal in comparison! I'd like a facelift C63 (2012 on) but budget is stopping me (only up to 25k to spend) so will settle for the pre-facelift for now.... Hopefully getting one this weekend

Chris