adding/removing users, etc. -- the kind of stuff you would do as a Plesk reseller?

For example, RS's Bargain Basement offers "AMD Duron - 1Ghz - Plesk - 20GB HD - 512 MB RAM", does that mean Plesk CP is already installed and ready to be manipulated with? Maybe I'm talking about managed servers, but then again, I could be wrong.

Any info would be greatly appreciated.

I think you would definitely be better off with a managed server because Plesk can't do everything for you. I am sure you will need to update programs etc. at some stage and you will have to use the command line to do this. So if you don't have much knowledge with *nix you should go for a managed solution. Also it might be worthwhile to buy yourself a book on linux and install it on your home pc. :D

I think Cpanel is better overall Plesk. My opinion atleast.

So which is easier to manage, has more features, and is stable? CPanel or Plesk? Also, if I select either one, will it be installed and ready for usage? Or do I have to install it myself?