No real contest IMO. Ive stopped playing the career mode, and now just play online. There are lots of options for tuning/modifying the cars.. Aspiration, Drivetrain swaps etc.
i must say i do enjoy forza 3. i love the downhills in the mountains. go into the tunnels in a lambo, turn up the volume and bass on my gamer chair and listen to that glorious v12/10.....