x86 is smaller. There is a better chance of getting drivers for your H/W (though that gap is rapidly closing). X86 is limited in the amount of memory it can address, approx 3.25GB.
X64 will address as much memory as you can throw at it - basically limited by available MBO. I've read where this limit is approx 192GB, and other places quite a bit more. Also, I believe x64 Win7 is larger (but not a lot) - the installation ISO, and the footprint both on disk and in memory.