What do you want out of the car? A Z4 is roomier and has a more modern design, while the Z3 is more a retro look roadster. I have a 98 M roadster and love it. Personally I am not fond of the Z4 look, but beauty is in the eye of the beholder. An MZ4 is coming out soon, that would be sweet. Consider that the Z3's are no longer being made.