Also, many flavors of UNIX are propriety and/or require expensive hardware platforms to run on. Linux will run on very ordinary PC components that you could purchase exclusively at "Worst Buy" (if you were a fool who was easily parted with his money). I've never run FreeBSD, but it is an alternative.
And, as previous poster mentioned, Linux is similar enough to UNIX that you can easily transfer skills. C, Shell Scripting, Python, PHP, and Perl are quite transferrable across *nix platforms.