The engines in each are the same; the only difference between the 2 engines is that the Bullitt has a better flowing intake basically. Both are 2V 4.6L. The Bullitt is rated about 10 horsepower over a GT...270 in the Bullitt vs. 260 in a GT.

You can do like I'm doing...I've got a GT, but I'm doing the Bullitt intake swap. Lots of parts involved, it'll run you about $800 for the intake parts and then another $800 for the install kit (includes bracketry, hoses, throttle body, new alternator, fuel rails, etc). Since the Bullitt intake is aluminum, it can be ported and polished out...I've seen people who have done this in their swap, and have reported about 15-20 RWHP increases.

But underneath the intake, the motors are the same. There are suspension differences between the cars (easily remedied as well), and some cosmetic stuff.

As stated above, if you're doing racing with the car, just get a GT and start with that. Could probably even find some better intakes out there (though they are few, and usually for hardcore racing purposes).