I came across this post on TaoSecurity today, with Richard Bejtlich's suggestions for an "Introduction to Unix." Like text editors, the discussion about which distribution of Unix to learn can be considered a "religious" argument. If you're a regular reader of his blog, it's no surprise he suggests FreeBSD. He does clarify and say if you're running a server, he suggests FreeBSD; and if you're running a desktop, he now prefers Ubuntu. And if you're still adamant about running Linux as a server, he suggests Debian.
My opinion differs, depending mainly on "why do you want to learn Unix?" If the answer is related to increasing your work/resume skill sets, I would have to disagree about using FreeBSD or any of its other BSD derivatives (Net, Open, etc.). In my experiences of being an admin or some other support role, I have yet to encounter a *BSD server. I'd encourage someone to use a distribution that they would encounter in a corporate environment. The Unix server OS's I've had to support have been Red Hat (now Fedora), Red Hat Enterprise Linux (also known as RHEL), and Sun Solaris. To avoid having to pay licensing fees, you could substitute CentOS for RHEL.
If I were asked the question, I would suggest a "major" Linux distribution or Solaris/Open Solaris instead. I would think their device names, software packages, and file system organization would help with familiarity when trying to translate the knowledge learned to a corporate environment. Although this may be an unfounded opinion, I also think that there are more support options and supported software using them, rather than using *BSD. I do agree with Bejtlich that if you want to run a Unix distribution on the desktop, to stick with Ubuntu, since it seems to "just work" when installed and there are less configuration headaches. Or you can just use a Mac if you want a Unix desktop (troll... and yes Mac could be considered a BSD variant).