Leopard. :-) You needn't even know if you're running 64-bit.
...
Windows XP x64 is hopeless driver-wise... And on top of that, even some of Microsoft's own stuff won't run on it, such as the Group Policy MMC.
I have 64-bit Vista on my work laptop... By choice, no less! Drivers weren't a problem and it's very stable. 32-bit support seems much better than xp64, 'though Audition 2 (pre-Vista, to be fair) was useless. And in general I gotta say I hated Vista with a passion at first, but I'm coming round a bit. You need to spend a bit of time disabling some of the sillier features and services, of course...
