Cool thing about the Xbox is that you can easily sync it with your home computer. Use it to listen to music on your computer hard drive, watch movies, etc.
That said, I don't really care much about the differences. I don't play nearly enough to work out all the nuanced differences.
However, if you like racing games, get the Xbox. Forza beats the everloving shit out of GT5. Then, you should also get Grid 2. Great racing game, more geared towards driving and technique than flashy cars, better simulation, but only if you have a wheel.
Speaking of wheels, get the Logitech G27. Friggin awesome. But, you need to put it on something. My suggestion is building one of these.
I built one in my garage out of wood (plywood and 2x4s). Used an old racing seat I had (Sparco Evo). Measured the dimensions from my racecar and replicated them in the "simulator". When I felt especially nerdy, I'd even stick on my race suit, shoes, gloves and helmet. Of course, this was before I had kids, when I could get a solid 5 hours of racing done in one sitting.