I have about a dozen machines with centos/cpanel and a few (decreasing amount as time goes on) machines with freebsd. If you are using cpanel, I'd stay as far away from freebsd as possible. You will not get much of a performance gain with freebsd, and you will encounter many more problems because cpanel seems to port its software to freebsd from the linux version.
Standalone as a server os, freebsd is great but i do not reccomend the freebsd/cpanel option.
That being said, learning freebsd isn't too hard. The only differences are in the ports system and kernel/make world aspects. You can learn everything you need from the freebsd handbook.
As far as rating it to a scale, I will let you interpret my ranking from my post as the number seems arbitrary to the matter at hand.
we presently run all freebsd servers at this point in time. I have in the past tried out a rhe with ensim pro on it which also worked nicely. I would though have to agree on the fact that cpanel will not run that great on freebsd. If you are looking at a decent control panel system with freebsd I would have to say go with DirectAdmin which runs near flawlessly on it. It really depends on your audience who you are trying to accomodate with services.
Overall FreeBSD is very stable when run properly and Linux can also be as well but may require a bit more on the hardening side of course.
I would rate FreeBSD stability wise a slight edge over linux however it really depends on the admin and what OS you're more comfortable with.
We work with a number of operating systems; each one has their good and bad points.
Since we currently do a lot of work with H-Sphere from http://www.psoft.net/ our experiences are bent in the direction of work H-Sphere works best on.
H-Sphere does work on CentOS, FreeBSD, RedHat 7.3, and RedHat Enterprise as well as Windows 200x (though not for the control panel, DNS services, or mail services).
That stated, H-sphere runs best on CentOS and RedHat in terms of the least problems encountered during upgrades, patches, etc.
FreeBSD does get a running start for hardening in that the environment is more secure out of the box than a CentOS / RedHat; but we find most open source developers working on a form of Linux with ports to FreeBSD either not coming or coming late.