64 bit mameUI only if you have a 64 bit OS (vista64, xp64, etc), which requires a 64 bit CPU of course. (All mainline CPUs are 64 bit capable now.)
So if you have the basic vista, which is 32 bit, you have to use ui 32, too. Also, you can run UI 32 on a 64 bit OS if you want, even though UI 64 will give you ! 20% speed increase.
To sum up, UI 32 for all modern OSs, UI 64 only for 64 bit OSs.