It has been a long time since I tried Parallels, but when I did try it, I hated it. Parallels tried too hard to integrate the Windows virtual machine in to the OS X host. What I mean is, it changed many of the default apps in OS X to open in Windows. It created an absolute mess on my computer. I don't know what other users' preferences are, but I want my virtual machine as sandboxed as possible from the host.
With Fusion, I didn't experience and of this "over integration" of the two operating systems. The functionality is still there but it is easily turned on and off by clicking on a checkbox or two. With Parallels, it took my hours to clean up the mess it created.
Secondly, I used Parallels with my Bootcamp drive and it installed all sorts of stuff in my Bootcamp drive that made it unstable when I tried to boot in to Windows rather than using it a virtual machine. I ended up restoring from a backup to regain stability. I idon't have any problems with Fusion using my Bootcamp drive.
Perhaps Parallels has addressed these issues by now, I don't know and will never try it again to find out. Testing it once wasted too much of my time. Fusion has worked very well for me for a long time and I don't have any reason to switch.