Haha, not quite Renault 3.5. Renault 2.0 with a variant Nismo tuned engine for a few extra HP's.
As for the differences between Forza 4 and GT5. I like them both for different reasons. And they are both fun for what they have on the positives side.
Car Graphics: GT5 (Premium Only)
Track Graphics: Forza 4
Car Selection: Forza 4
Track Selection: Forza 4
Car Customization: Forza 4
Controller Physics: Forza 4
Steering Wheel Physics: GT5
Immersion: Forza 4
Sounds: Forza 4
Online Racing: GT5
Just my opinions though.