Go with Windows 7 64 bit. It has matured and is pretty stable. Also easier to work with, out of the box.
Also, depending on what you use your pc for, you might want to go with Windows 7 Home Premium 64 bit, but it will limit you to a maximum of 16 GB total ram
once you hit Windows 7 Professional or higher, you break that 16 GB system Ram threshold... and can go up to 192 GB
Windows 7 Home basic is limited to 8 GB total for the 64 bit version
I do not recommend any of the Windows 7 32bit OS versions....... max on RAM limit on these can be either 2 GB or at best 4 GB
just a FYI
TC