Ditto to what DinoGTS said about the looks. I love the look of the Z3 and hate the looks of the Z4. I own an 02 M roadster and my only complaint is the handling and lights. The handling is excellent, but not enough for the way I drive the car. The trilogy from Strong-Strut solved the handling problem, and I am still working on a way to get Xenon HIDs given the Gov's ban on aftermarket HIDs.
I will keep the M roadster until I cannot get parts for it anymore. The 02 was rare (about 400 made for the US market) so it will continue to appreciate in value anyway.
Talking to BMW execs at an auto show left me with the impression that a Z4 M is at least 4 years off, if ever. Without the M power, enthusiasts of spirited driving will avoid the Z4 and pay the price for the 01 and 02 M roadsters and coupes.
It must say something that an 02 M roadster brings more money than a brand new Z4.