When the box gets sluggish check for Spotlight indexing. Easiest way is to hit the magnifying glass upper right in the menu.
If not run Activity monitor, sort by %CPU, see if you have a process that's being a pig.
Not a lot of difference between Parallels and Fusion. They've both been around long enough where sometimes a version of one is better than the other.
Years ago I used Parallels but there were a couple of crappy versions and I switched to Fusion.
Currently Parrallels has the advantage.
VM Benchmarks - Parallels 10 vs. Fusion 7 vs. VirtualBox