If you want to start off with something on a more intermediate level, I suggest Debian. In my years stuck in Ubuntu and derivatives I hardly learned anything, until I switched to Debian.

Well, if you do a custom net install you can learn quite a bit.

At it's core, Debian IS Ubuntu with less hand-holding. (Ubuntu is based on Debian)

That helps out with all of the Ubuntu documentation out there. But remember that it's only mostly compatible.