Parallels' Coherence works better than Unity and it has more supporting features (SmartSelect), but at least in the betas, the stability of the virtualization itself has left a lot to be desired. When I sleep my iMac (Mid-2007), any running Parallels VM restarts. This has always worked flawlessly in VMware in Windows and OS X, even in betas. And whenever I try to suspend my Parallels VM, it is unable to wake up.
Also, I've had two times where my system has locked up because of Parallels.
In other words, Parallels' basic functionality - the part of it that should remain mostly unchanged - is the part that isn't working very well. I'll probably end up moving to VMware in a few weeks, and hopefully they'll release a new version that catches up with some of Parallels' features pretty soon.
I have no doubt that VMware is much better at virtualization. They're also much better at releasing products that work as intended.