From what i gather, the mach1's set up is better suited for drag racing/launching. Even though the spring rates are the same, the only difference is the custom valving in the rear?
The Bullitts set up is better suited for corner carving and stability. I've read the the Mach's suspension is softer, and you feel less of the jolts and such on the road. The Bullitts suspension communicates with the driver better. As a Bullitt owner, i will admit that it does ride alittle hard over bad pavement, but it definitly handles noticably better and with less roll then GT's. I havent been in a Mach1. Also, the Bullitt is lower, which again is a plus in my book.
So you have to look at it like this: if you want a softer, more comfortable ride that is lower then a GT, and better for straigh acceleration, go with the mach 1.
If you want a lower car, that handles better in the twisites, rolls less, is a little harsher, and not AS good in the straight line set up, go with the Bullitt.
Either way you cant really loose

Mike