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