If you just want to learn, install the desktop version. That way you can learn both the GUI and the command line: you don't have to use the GUI if you don't want to.
I don't recommend Ubuntu. The documentation is poor, they're always trying to make money out of you, and they conceal that money in a tax haven.
Three distributions that stand out for the amount of documentation from which you can learn are Debian, Arch, and Slackware. Arch is difficult to install, but Bridge will do the job for you. Slackware is short on software, but Salix gives you it with lots more programs. Read the reviews listed at Distrowatch and see what you think: