You should have specified which Z4. There are three different models: 2.5, 3.0, 3.2 being the M3 engine.
Here are some 0-100 time of the manuals from the Z4
2.5 = 6.5sec
3.0 = 5.7sec
3.2 = 5.5sec

My choice would be the s2000. I just love it quick, stylish and the Vtec.