If you are familiar with the RedHat environment, then centOS is a solid choice. I have moved from RPM based systems a long time ago to a Debian based, so that is what I go with on all my servers/VPS's. It's personal preference based strongly on what you are comfortable with administering. centOS is requiring me to learn a new process for installing/updating the software (I run it on a local server at my house and on a couple of the VPS's I have set up on one of my dedicated servers).
Just stay away from Windows IIS.
No. XenForo will after all run on any OS so long as it has php and mysql.
As personal preference, I always recommend Debian. I find that out of the box before you even change anything, it uses fewer resources by default. And this continues to hold true even after you add things to it or tweak it to your liking. It is also very user friendly to setup. (even for newbies).
If you prefer a control panel, there is even Direct Admin (link). Which has most of the features as cPanel. But uses fewer resources, is kept up to date (quick to adapt new tech), is customizable, and cost less. Best of all it supports multi Linux distros so if later you decide to change OS, you can do so without being locked in.