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.