My advice is Windows 7. That being said, I am currently trying to figure out a way to salvage my installation after I cut power due to a lock up after I did something stupid. I pray that I can somehow recover the files and then reinstall.
I run XP with no problems (knock on wood). Be sure that 7 is supported by VMW - if I was starting over I'd probably still use XP due to compatibility issues with software I use on XP.
I wouldn't board the Windows 7 ship just yet. Microsoft and VMWare will inevitably need to iron out some kinks on their software (the former probably more so).