Balls, in this case I agree with you. I am sure the Z4m will top the test track over a 135i. I cannot comment on the performance and "feel" of the Z4m as I have not driven one. I sat it in but that was in a showroom, I'm 6'4 and I'll be honest I felt very awkward tring to get out afterwards (it wasn't a gracefull exit)
The 135i is marginally better to get out of, but not much.
Perhaps one day I will drive one and then I can comment on it in more detail.
With a wife and kid it's never going to fit into my lifestyle, but more shall I say an individuals car, some say a drivers car. Then again if I had the real bucks, was single and mad, my personal choice would then be a TVR Sagaris for personal indulgence.