Don't buy it on xbox if you have it on the PC already.
Instead save that cash and hunt down some upgrades for your computer.
Remember that you can trade in some parts of your computer too, so that can count towards it.
Honestly, using the money to upgrade your PC makes far more sense than buying it again on the 360. Not only do you get to enjoy the game you bought, you are investing in your other games' performance too.:)