I am a former owner of Z4MC, E92 M3, E46 M3, and E70 X5. I can say the Z4 was a great little car for what it was but me being tall it just took its toll on my back and thus had to get rid of it for that reason. Moved over to E92 M3 and never looked back. At first I didnt like the E92 M3 in stock trim. Looks horrible. Now after $10000 in aftermarket parts on the M3 its now looking how its supposed to look. lol
I would have considered an E89 Z4M but honestly with the room in the M3 and more power, it was the right decision to move on.
All great cars none the less..