From a user or novice point of view, Plesk has nicer interface, easier to use and that is pretty much it.
From an experienced admin who needs to do serious work on a server and customize the server to their specific needs, Plesk falls way short. I hate the PLESK structure on Unix, it lacks the simplisity of cPanel backup tool which will save any system admin alot of time in recovering accounts.
I still believe for an experience admin, Plesk is behind even with their 7.5 version
My vote goes to cPanel though I wish they focus more now on perfecting the panel, adding more cluster options than adding extra features.