What are the main differences between the three world editors? Is the DX 10 or 64 bit versions simply for editing, or does it output different files when saving worlds? (ie. use the normal world editor if your target is 32 bit windows xp, or use the dx10 editor for windows 7 platforms).

they produce the same files

it's means differences world editors use differences System resources?
for example:
use winxp 32bit can't use world editors DX10 or 64bit
use winxp 64bit can't use world editors DX10+
use win7 32bit can't use world editors 64bit
use win7 64bit can use all world editors
but the all world editors produce the same files

Are there any real benefits of using the 64bit WE versus the 32bit?

More memory available when building your worlds, and usually world editors takes up way more memory than the game does.