Sorry I've been away from this forum for a little while.
IMO a Comparison between Forza and FC goes a bit like this.
graphics
Forza...........7/10
FC...............5/10
physics
Forza...........7/10
FC...............9/10
AI,
Not sure, I play mainly online, but my limited pre patch experiece would rate the AI higher on Forza than FC, don't know now it's post patch.
wheel control
Forza.............4/10, MS Wheel and Logitech Drive FX
FC.................9/10, Logitech Driving Force Pro
online
Forza.............8/10
FC.................3/10
I enjoy both games, but the simple litmus test is to think.
"Would I be happy if I bought the console specifically for the game?"
Forza2.........................Yes
...................I did and I am.
Ferrari Challenge.............No
....................I only bought the game because I had a free PS3 sitting doing nothing, GT5P was a rubbish experience so I put a lot of hope in FC, it's OK, but I don't believe it'd be worth the outlay for a console just to play it.