"Solely for track purposes" to the extend of fitting a roll cage, striped all rear seats and door panels, upgrading suspension, brake system, running w/o cats, slick tires and transported in trailers? Or race on weekends and still be able to drive to nice restaurants on occasions? For street and occasional track used, the M3 is hard to beat. If you are talking about a purposely built track car, there are better cars out there than both. The 370Z is a mediocre sports car and are just middle of pack at track days.
E92 M3 DCT, Cayenne Turbo S, Ferrari F430 F1
911 Turbo (996), C32 AMG, S2000, (E36) M3, Supra TT, MR2 Turbo