There are some other issues here besides the race. The Mach is a cool looking package, it screams for attention. My neighbor has a blue one and a blind guy could find it in a WalMart parking lot. It's the gunslinger of the two and everyone is coming gunning for you. If that appeals to you that is a big plus, but if you would rather slink down the road and pick your battles when you feel like it, GTO.
Auto vs. Auto the GTO blows the Mach away. Fords 4spd auto sucks the life out of that car.
The GTO handles a little better, but is worlds ahead of the Mach in fit, finish, ride and appointments. The GTO compares to BMW and Mercedes. As far as that goes and the Mustang compares to ..... a Cavalier???? LOL. Just kidding they did improve the interior of the Mach over the standard GT, but the GTO is still a couple steps ahead.
GTO will get better mileage on the highway, but about the same around town.
If I had an 03 Mach, unless it was an auto, I don't think I would spend money to trade for a GTO. The Mach would really have to be giving me troubles to make that move.
Let us know which way you decide, and thanks for asking.