Which Zed do you have? A 276bhp one? In which case we have already established that the z4m with an extra 50bhp is bound to be quicker. But when you are talkin about the 313 350z or the 370z then there is very little in it. My zed for example, if you floor it in 2nd, will literally throw you back in the seats, the back end squats down, loads of grip and the revs hit 7500rpm before you know it, and mines the roadster so heavier than the normal 313 coupe.
The z4m is a very nice car, but I think at the end of the day its all about personal preference as to which one people like the look, feel, drive and the costs of.
I think you need to take off those rose tinted glasses
The MY07 zed has more BHP than the MY03 cars but is heavier (yours even more so as a roadster) and has less torque. The on road difference is negligible even if the MY07 'feels' faster in the way that it delivers it's power, in real terms there's virtually nothing in it.
The Z4M has the E46 M3 343bhp engine and is considerably lighter than the zed, hence why in performance terms it will knock a zed into a cocked hat. The ride may be compromised and may not be as comfy as a zed, but let's not pretend that it's not quicker than a zed just because you don't like it.