My vote would be for the Z4, granted I am a bit biased towards BMW, but it is a great deal newer so any original rubber in the bushings will most likely be fresher and you will have a good deal more power with the N52 to pull out of corners if you are new to track driving.
I definitely recommend at the very least a good inspection of all suspension comments and upgrading to some
High Temp Brake Fluid and more
Track focused brake pads as street compounds will fade after a few laps of heavy braking.