OK, This is an interesting question, lets look at some facts between them all:
Mustang GT: Approx $26,000 - $30,000
1) Great Car
2) Strong Motor
3) Supercharger will bring you to 450 HP safely, any more you
need to upgrade the motor.
Roush:
1) I may be biased but Awesome looking cars.
2) You dont see them two often.
3) They come in 3 stages depending on your budget.
a. Stage 1: Looks. Approx ($38,000 - $40,000)
b. Stage 2: Looks and Suspension. Approx ($48,000 - $50,000)
c, Stage 3: Looks Suspension, brakes and
supercharger. (Approx $58,000 - $60,000)
GT500: ( Not quite sure $ wise)
1) In My Opinion OK Looking car for the money.
2) Awesome Motor
3) Awesome power
4) Can upgrade to 600 HP with a $5000 supercharger upgrade.
and everything else is done. Woo Hoo

So breaking that down you can have a:
1) GT: Make it look like whatever you want it to and increase
HP as you please to a limit and would probably the cheapest
option.
2) Roush: I cant believe I am going to say this but besides
nobody having an idea what a Roush is ( I know, I Know,
everybody and there mother asks me what a Roush is, Go
Figure) I would not buy a roush, It is great that it is a Roush
and you dont see too many people with them, but it has the
same limitations as a GT but a heftier price tag.
3) GT 500: As you know in my opinion it is an ok looking car for
the money. But you get the prestige, The Power and it is
already done, unless you want to jump to 600 HP.
Conclusion: If you can find a GT 500 for a decent price tage buy it save yourself the heartache of having to give your car up for upgrades and the GT 500 Comes with a warranty. I know this was long winded, I just felt it was nessesary.