you should seriously drive them before you sign up for one of either
I have a Z4 3.0 (from back when the 3.0 was the big motor) and have taken it to the track a few times and autocrossed too..
when I test drove the M roadster I really, really, really wanted to like it
The power in that car is AMAZING
the steering wheel was too fat for my tastes (minor)
the handling was twitchy, spooky, and a bit unsettling at the limit (major)
I really wanted to like that car, but it just didn't seem to be a polished product. It seemed like the car was just a hodgepodge of m3 bits tossed onto the Z4 to hype it up a little bit. Early reviews from mags, and performance numbers would tend to back up my feelings from the test drive. I had a friend (boxster S owner) with me and he drove it too and we both came back with the same conclusions.
very impressive power, not easy/fun to drive near the limit
You really get the feeling that they're handicapping that M roadster and coupe so that it won't just destroy even the new E90 M3 in every measureable performance measure.
After test driving the M roadster I had zero desire to upgrade.