The GTO is a little more practical than the Mustang since it actually has a back seat that won't cause crippling injuries to adults trying to sit there. It also has, lets face it, a better engine with about 100 more HP than the Mustang. Its straightline performance is supposedly better than the Mustang, and they both handle well enough that you won't know the difference unless you're a professional driver.
On the other hand it costs more than the Mustang and--likely why we don't see many of them on the roads--it suffers badly from a very bland appearance. It looks more like a Japanese import than a Pontiac muscle car. With a car like this, the look is just as important as how well it actually drives, so that is a serious drawback.
Overall, personally I pefer the Mustang just due to the more distinctive appearance, but the GTO is probably the better car in technical terms.
J_A_B