Parallels lost me as a customer last year when they were charging full price to existing customers to upgrade to the latest version (I don't mind paying for the latest and greatest, but I feel they should have given a discount to those already using their product). This wouldn't have been such a big deal, but version 7 was buggy in Mountain Lion - so you were pretty much forced to upgrade Parallels, or downgrade OS X. Anyway, VMWare was offering a discount on their software to existing Parallels customers, and it was cheaper than what I would have paid to upgrade Parallels, so I made the switch.
TL;DR: mostly a grudge I have with Parallels.
They're essentially the same and have most of the same features - both great software. That's why I suggested to download the trial versions and play around to see which you like more.