If you're not very happy with your M3, go for it. Me......I've had my M3 for about a week now and I'm crazy about it. I plan on keeping it for at least 5 years, if not much longer.
The C63 is an amazing vehicle though. I would've got it if weren't for the price with all the options I really wanted. If you do decide, make sure it has the AMG Performance pkg. If not, I guess it's not a big deal.