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.
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