From what I've come to understand, you won't really gain anything if you install 64-bit version in terms of gameplay; because the game(s), unless they were written to support 64-bit system, won't benefit from it.
Another thing to consider is how much space you're willing to dedicate to your partition. If you don't have much to spare, 32-bit will be your best bet because it takes up "only" about 8GB, while 64-bit takes up almost twice as much.
I was in the same dilemma as you because I use BootCamp mainly for occasional gaming and to run an online streaming program called PPStream (which I would watch by running Parallels/VMWare), so I figured I wouldn't really benefit by installing a 64-bit version.