Personally I think debian is the best server OS to learn. It's sufficiently similar to ubuntu that you can get away with either one really, they aren't very different as ubuntu is based on debian.
I'd recommend Debian as the uncontroversial Linux server distro.
But for someone who might have some Ubuntu experience and little other Linux experience, running an Ubuntu server might lead to fewer surprises. Ubuntus do ship with things that Debians don't have. I can't name any, since I don't use Ubuntu, but they're the things about which you either say "how nice!" or "how dull!"
But for someone who might have some Ubuntu experience and little other Linux experience, running an Ubuntu server might lead to fewer surprises. Ubuntus do ship with things that Debians don't have. I can't name any, since I don't use Ubuntu, but they're the things about which you either say "how nice!" or "how dull!"