I've not used Plesk except to play around with it on a server I once had, but I love Cpanel. I've always made sure to stay in the "Stable" tree in case of bugs. You'll find if your're offering linux based hosting that offering Cpanel will drive more customers because of the ease of use of the User panel.
I agree. I think cpanel is great. There are so many things available within cpanel. Also, I've found there is much more flexibility as to what you can do server-side.
I prefer Cpanel because of transfers, but it does tax the system more with a lot of unnecessary files running. Unless you know how to tweak it you will find that it can slow you down.
Both are the best control panels available today but I have sticked to cpanel for some years now and more so because cPanel is the more popular control panel out there.