Depends on what you want to do.
The GT500 is going to be very fast in a straight line. But the Ford 5.4 mod motor is a giant heavy iron thing hung way out in the nose, the weight distribution on those things is ghastly and the Mustang chassis is nowhere near as good as the V.
The GT500 will be a great straight-line warhead and a joke any time the road turns.
I'm just wondering why the original poster would be comparing these two vehicles, seems to me that if you can live with a Mustang you're not going to buy a sedan, and if you want a 'balanced' car the Mustang's not going to have much appeal.