I have tried Parallels 9, and VMware Fusion 6, and settled for Fusion.
I had some strange stability issues with Parallels, but absolutely no such issues with Fusion. These might have been caused by something specific to my configuration, so YMMV. I reasoned that the issues would eventually get worked out either by updates to OS X, Parallels or Windows. But Fusion worked right away, so I chose it instead of waiting.
I also didn't like Parallels way of putting shortcuts of Windows application to my dock or anywhere else by default. Or the strange glitch of having the red Parallels logo appear on
every single OS X app icon in Spotlight (because the Windows app shortcuts were also indexed, I had to always look carefully not to launch a Windows version of any app I had in both OS's).
Performance seemed quite similar. Parallels would have offered support for DirectX 10, but I had no need for that.
If you want to find out more of the performance aspect, Ars Technica has a good review with benchmark data:
The latest virtualization showdownParallels Desktop 9 vs. VMware Fusion 6