i have both and i can say that the FGT is much faster and more useful, but harder to control out of corners (300 torque in a car that has only 545 kg weight). but, my speed 8, with a turbo, needs no mods to the transmission to hit top speed, at top speed on la sarthe, the engine was just under the limiter of 7000. the bentley sounds like the dull wush of the r8 race car, although the fgt isnt much better (revs sound like 6000, when redlining 1 at 17,500). FGT has more grip, but be careful, because the power can overwhelm the grip fast
Conclusion: FGT is more powerful, lighter, higher revving, butter sounding, grippier, but needs mods to the transmission to hit top speed and is less fun than the speed 8.
Reccomendation: If you just want to win races, get the FGT. If you like going to Online Lobbys and just playing around (like me) get the speed 8.