My dad's looking at getting a Shebly GT500 and found a good looking one just down the street at the local dealer.  It's a used 2010 with very low miles.  I like it too (it looks uber mean in the all black), but told him he's crazy if he trades the viper in on it.  Let's compare why it's crazy shall we:  

1.Horsepower - Viper has 600, GT500 has 540

2. Viper quicker in the 1/4 by about 1.5 secs

3. It's a freakin rare Viper and there are a ton of stangs on the road