Here in Turkey we have Corvette's than Z4s and in every track day there are at least a couple of vettes.
As everybody who replied stated the choice will depend on what you expect
C5 is not a match for the z4. However, it will pass the Z in drag racing.
C6 on the other hand is a totally different beast. It handles like on rails and very powerful. Only the Z4M can compete with it. 3.0i or 3.0si has no chance against it. Especially I think z06 is the best valued sports car and a Ferrari eater.
As all other American cars the built quality and interior design will not impress as much as a BMW. Perhaps the ZO6 is one of the few cars that I would change my Z4 for.