911 turbo vs 07+ Z06 vs Viper
Porsche is going to be expensive to maintain, plus with $50k you can either get the slower 997 Carrera or a quicker, but outdated 996T. It's still going to be the slowest car out of the three.
Stock for stock an SRT-10 Viper (1s and 2nd gen.) is going to be a little slower than the Z06, but it is also rarer than a Z06 and gets more attention.
So really depends on what you are looking for.
I'd throw the Porsche out unless you somehow find a 997T under warranty with your budget.
GTR is nice too, good potential, but mods cost $$



