Depends on what you want.
If you're a person who likes family games that you and your family can play together, I highly recommend the Wii because of it's large margin of E and E10+ games like Wii sports resort, Wii ski, and other games. (I don't know many because I own an Xbox360).
If you're a hardcore gamer teen who likes M and T rated games, I highly recommend the Xbox360, because it's also great at what it's going for. Just look at Halo 3, Call of Duty 6, Gears of War 2, and Left 4 Dead 2!