Couple of threads like this already but i'll bite once again
Like AK said.. the 135i was my other tough decision.. Almost fully loaded 135i (minus Nav) the car was 5k to 6k more in price. The car drove much smoother, and definately had a lot more torque down low but didn't feel sporty to me and definately didn't handle like the Z. The savings in money and the Z being a much more polished vehicle and much more track worthy is why I chose it in the end. Plus I get horrible boost fever and would blow up the 135i in atleast 1 years time trying to get that extra HP.
In about a year i'll see what the boost market is like for the Z and I can take the 5 to 6 thousand dollars I saved from not buying the BMW and put a supercharger/TT kit on my Z.