You need to do much more research, they're completely different cars.
The C63 is track focused, incredibly powerful/torquey and with an earth shattering sound. But IMHO its not an everyday car, it's just too uncomfortable (hard ride).
The Z4 (e89) does not have an M version (M sport pack is a token mainly superficial thing) and is not track focused its an everyday car you can also have fun with.
Black Sapphire BMW Z4 sDrive35i
Mods: HiFi Tweeters Stubby LED Halos & Front Indicators DVDInMotion SmartTop 313 BMW Performance Alloys LCI Repeaters http://goo.gl/PF4nSz