I would get the Camaro. Ford hasn't made an NA motor with nuts since 1995. Plus I have personally witnessed a SS tear up Buttonwillow (tight and technical) and go turn for turn with an Elise.
The Mustangs may have more refined interiors, but they are SLOW. I never met anyone with a modular motor GT who ultimately wasn't just disappointed. GT500 are fast for drag racing ONLY. They get them on a race course and the Civic Si guys eat them alive until they finally pit in due to overheating.
Here are some other suggestions:
Solstice
: great overall setup and cheap to "reasonably" increase power with the turbo. Suspension architecture is great, but needs stiffer shocks and springs to take advantage.
BMW 135i
: power to wieght ratio is good. Great torque! Strong after market support. Downs: not american, NARROW track means you better know what you are doing.
Genisis turbo coupe: Cheap, but it's also cheap
.
Evo
: It drives itself. Pretty much the only "import" car thats actually as fast as the ricers think it is.
370z: Has nuts and handle great, but they run out of breath. Very limited performance envelope from the VQ without dropping big dollars . JDM Mustang