I think I would go with the C63 here... Because though I've always loved the M3, I think this one isn't quite as focused as the earlier ones and on one hand that is very good because it makes it more useable in everyday life, but on the other hand, it just feels a little less like the M3s of yesteryear, especially the E46 which was just a blast to drive!

The C63 is nothing like the M3! It is just an autobahn rocket which just happens to be very capable of eating up tight corners(though maybe not as capable as the M3) while still maintaining a high level of luxury and that is really what you want in such a car! Because though these cars are sold as performance cars, very few people buy them to use them as pure driving machines just for fun. Because if they buy one of these, they are most probably to use it as an everyday driver AND a weekend-fun car and I don't think the M3 is very easy to live with on day to day basis! And if you can afford to buy one of these cars as a trackday car, chances are that you won't buy either, you'd buy something smaller and more nimble or something like a GT-R or a 911...

The only argument I can really find for buying an M3 is the huge numbers of bragging rights (something like 100.000). I know it's a harsh statement, but that is how I feel... Maybe I'm just getting old, or maybe it is the M3 which is trying to play grown up without really succeedeing.