ben g said:The S2000 is a better car in terms of being a proper sports car, from what i've read. The Z4 is a nice everyday car that suits me (could do with more power though) but will never be the ultimate driving machine.
I've had them both and I wouldn't say either is a better sports car, quite different (I prefer the Z engine, S2k was annoying and thirsty) however the S2K had hugely superior build quality ,so they will likely remain in better condition for longer and will be increasingly relevant as the cars age.