Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
To me, Parallels seems geared to integrating EVERYTHING windows has with OSX... No, I don't want that let it run self contained. I use VMWare for this reason. Also Parallels has a very sketchy history of pushing ads for upgrading into their software. Scumbag company.

Well, I'm glad you said it. Basically, though, they both do the job fine. They just do it differently. If you are a non-savvy user who needs to run Windows on your Mac for whatever work related reason, Parallels will do it for you without any real effort or skill requirement. Fusion gives a more savvy user more flexibility to do what they want / need to do and nothing more.
 
I would recommend to try Virtualbox first. It's not quite as polished as VMWare/Parallels, but for simply running some Windows software it'll do fine. Performance improved significantly with version 5 (although I haven't benchmarked it against VMWare recently). It's also well-supported for development use (Vagrant etc.).

If you find it doesn't meet your requirements, you can still switch to one of the paid solutions (personally I prefer VMWare).
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.