The Caymans are some amazing machines but I'm guessing the 370Z would have a higher reliablity rate and lower repair/maintenance costs and would be the one I'd consider of the bunch.
I'll be keeping my eye on the 370Zs to see how they do over the next several years in terms of actual real world reliability and used prices. They're right in there with my default MX-5 (eyeing the 2009+ NCs), an R56 MCS, S2k, and maaaaayyybe a C6 Corvette, probably a base model with at least the Z51 suspension setup.