Plesk= For the more advanced person that knows what they are doing. The GUI isnt as pretty, but it ca n gets things done faster if you know how. If you know how to use it, its a dream.
Cpanel= For the average user. Not to say that people who use Cpanel are dumb, but its alot easier to navigate and view what you want to do by buttons and images which makes it very easy. Cpanel is very good looking, and with fatastico, like nic said, it is very easy to do things that beginners cannot do on their own. So, its just more convienient and quicker for those who arent familiar with plesk. I have to admit though it does have a handful of more features :D