From what I have read, they both will get the results you want. The flex will take less time than the others. That's the main reason why I am going with the flex.
1990 Mustang GT
306, typhoon intake, E303, stock heads, 1.6 FMS R.R., 70 T.B., 80mm PMAS, 42lb injectors, 255 fuel pump, adj. billet regulator, MSD dist., coil & 6A box, BBK shorties, o/r x-pipe, Spintech Pro Streets dumped, T-5, Center Force Dual Friction, 3.55's, S-Trim... 396hp/398trq