i have been playing both, and i already knew which one i liked better
Forza 3 BY FAR!
Shift's physics are whack, the game can be fun, and looks good, but F3 all day!
considering i have not seen anyone from CR with Shift even in their gamerscore catalog when i'm online, if you play Shift you will most likely only find my Gamertag with all the records on there, not that it would have mattered, go look at the F3 demo, i own all those records as well