Test-driving a GT500 will be darn near impossible unless you can find a used one, or you've got a few thousand dollars to bribe the dealer with...
I hear where you're coming from, though. The king Mustang is a very hot package. It's packing 540hp, big Brembo brakes, a gussied-up interior, aggressive looks, and competent handling. The engine comes built from the factory, so it's not out of the question to easily coax another 200-300 hp out of it. Beware though, that's a realm for dyno queens, because you'll melt your tires every time you twitch your right foot...
Negatives, as I've read it, are an inability to launch consistently (see MotorTrend's 12.8 1/4 mile time...not typical...but those guys do this for a living, and that's the time they got), and skittish handling around curves over rough roads.
The SS, has 430 angry horses, an Independant rear (more competent cornering), mildly lighter weight, more balanced weight distribution, Brembo brakes, a nice interior (a 9 if the Mustang's a 10, but go sit in them both and see for yourself), and looks that'll kill. It gets more consistant runs. Usually at or just above 13 in the 1/4 under average drivers. The LS3 has roughly a 650hp ceiling while remaining reliable, and Pedders can hook up your state-of-the-art suspension system so it can out-manuever a Corvette.
The only "negative" to the car would be its tendency to under steer at the limit. Something you'll only notice if you're racing. Be that as it may, it's easy to fix.
In my very humble opinion...you can get GT500+ performance out of an SS for about the same $$. And keep the look we all fell in love with.
That's about all I've got at the moment...someone might spark my memory later on. Hope it helps!
When are you thinking of buying?