Forza is simply unbeatable on the 360 atm for racing. The game is what you make of it. It's more of a mmo then anything. Whether it is organizing specified racing events, tuning companies battling out to get the top times or paint companies releasing new fantasy or race replica paints. If you can get into the community with Forza you will have a game that will last pretty much until Forza 3 comes out. I have easily invested 1000 hours into Forza since the game was released just over a year ago.
I can agree if you prefer to play solo i would suggest GRID is a great alternative but if you can get a bunch of mates together in a racing team then there is really nothing that can beat it atm.