I implore you guys to check out this NASIOC thread:
Track car: the sti just won't do forever... What to get???
Lot of Miata, S2000, and Corvette discussion in there.
Most people voted for the Corvette Z06 for many reasons: power, handling, parts availability and accessibility (available at any local NAPA) and relative affordability, and reliability.
There were some definite Miata and S2000 enthusiasts there too though. Good info in that thread.