Letsee, judged on experience alone.

Car Physics - GT5
Tire Physics - Forza 4
Car Feedback - Forza 4
Car Models/Details - Forza 4

Singleplayer - Forza 4
Multiplayer - Forza 4
AI - Shift 2

But frankly, all these points are moot if the rest of your friends are on GT5 and you're the only one on Forza 4.