Z3
I prefer the Z3 for two main reasons: A) I have found it to be more fun, and I drive both almost every day. The Z4 just seems...bland. Not stylewise, but handlingwise. B) The Z4 only came out about a year or so ago, and being a technician, I drive MANY more Z4's than I do Z3's. And you know what that means...
Basically, the Z3 is sort of like a combination of the E30, E36, and with the later models, E46 technology. Oh, and I might be a little partial to the Z3 anyway.
Do not get me wrong, I love them both. I would not at all mind having both in my driveway, but I think the Z3 would get a few more miles on it.
Just my $0.02,
Sturm