If you know Linux or Unix, you'll likely be able to do most things in the other OS. I would think that if you want to Learn Unix, FreeBSD will be just fine. If you want to learn Linux, you'd want something like Slackware. In my opinion Slackware is about the most pure Linux OS out there. But then again....I'm a slacker
All in all, you can't do it over night so spend some time learning both Unix and Linux.
It's all good stuff
Peace bro!