Each control panel has it's merits of course but, i personally would say that both Plesk and cPanel are equally easy enough to use but cPanel is more standard as mentioned above, as well as the setup of the control panel out of the box is easier as well.
All tho Plesk might not have as much explanations and choice of configuration for the software you will be using it is easy enough to use

and it is also a parallels product and integrates with the parallels infrastructure manager which we use for the Virtualization management and the framework of VPS creation and maintenance within the company (
http://en.wikipedia.org/wiki/Virtualization) and might work well with the load balancing and performance better than cPanel.
On the other hand cPanel has released a full fledged installer optimized for VPS servers which is equal to the above performance to a certain extend.