If you're most interested in speed, not necessarily reliability, go with the 5.0. I feel that the 4.6 is a superior engine in terms of longevity, but it is more expensive to modify and suffers from a breathing problem. Since you will be purchasing an older car, keep in mind the 5.0 will probably have a few reliability issues, since it will be both older and have an engine that may not last past 150k miles. If I had to do it again though, I would probably get the 5.0 over my 96GT.