Unless there's a connection through to the internet, there's very little to choose between CentOS and Debian/Ubuntu Server.
CentOS is slightly cleaner on installation which means that you have to add all the services you want, but it also means there are fewer potential points of entry/security failure into your system. You'll also have to do a little more configuration.
Debian/Ubuntu are hardly wide open, however, so if you prefer those then go for it. You can spend a similar amount of time hardening Ubuntu as you would adding the services you want to CentOS
CentOS/RedHat is the server of choice for most enterprise installs, so worth using just for the experience if you do/intend to go into this kind of work.
In my opinion, there isn't enough to separate them - especially if you're not public facing. Either way it sounds like you're not too experienced with Linux - consider using WebMin/VirtualMin and using them to administer the service and virtual hosts.
