It kinda depends
Myself , I have used windows 7 64bit ultimate since msdn released it
Iam going to give windows 7 32bit a go now , as I want things like
my sound card to work properly , pvr / winamp programs are struggling
with outputing sound correctly , I have tried win7 64 on both my work laptop
and my PC in the house (totally different sound chips) ,similar problems with both.
I have tried pretty much every driver for both cards but the problems persist
Maybe closer to "offical" release time they will have things fixed , right now I have gone back to ye olde trusty Xp32.
I will take working system over a pretty UI any day