I haven't had direct experience with either of them, but our Mercedes-Benz dealer once walked us around a C63 AMG they had for service, including firing it up. Boy, did that sound good...

I think that both the supercharged 5.4 litre and the normally aspirated 6.2 litre are two of the most charismatic engines Daimler has ever made. You can't really go wrong with either of them.

(I personally have soft spot for supercharged engines, but I like big normally aspirated ones as well. I'd have a tough time deciding...)