Are you joking
This is as apples to oranges as you can get IMO.
Having said that, if you don't need 4 seats and have the dollars to deal with out of warranty repairs, I would pick the C7 Z06 without any doubt. It is super car performance in all respects... put it up against virtually all cars in the $250K price range and it's right at home. It will simply embarrass the M2 in any and all performance measures and is one damn sexy car to see on the road.
The M2 is fun and sport and more practical with back seats. That's where the comparison ends.
I love the M2 and can't wait for mine. However, it's a compromise car... if I didn't care about needing a back seat with kids, I would be in a C7 of some sort... or a 911 something.