CentOS, Debian or Fedora for an Ubuntu User? [closed]
If you know Ubuntu, Debian will be by far the most familiar. There is very little difference between Ubuntu and Debian on a server. CentOS and Fedora will require you to learn how to use RPM and will have files in a different place. For example, they do not have /etc/network/interfaces and Apache configuration is in /etc/httpd. They're just two I can think of off the top of my head.
IIRC, Ubuntu is a Debian derivative, so maybe Debian?