totally based in my experience. I could give a generic answer as "important is to get an OS which you are most comfortable installing", but when we are speaking about, for example, VPS, or any other kind of "server as service", that install is just matter of a click. I would use another factors to decide it. I like to use debian and ubuntu. Here are my strong points about both:
Easy to upgrade, update and compile new packages. If something don't compile in debian, it don't compile at all :-)
Easy to be up and running with a new deamon
apt-get && aptitude are great to install and to solve dependencies
Good as servers, good as desktops
Said that, i manage 4 servers, running debian, apache, mysql, passenger and i' really happy that i dont have to touch CentOS anymore.