I have used a variety of UNIX flavors including Solaris, FreeBSD, HP-UX, Linux, and OS X. I can tell you first hand it matters little what flavor you use if you are new to the UNIX world.
Both Mac OS X and different distributions on Linux provide bash. bash is the Bourne Again SHell. You can get to know the nuances of bash right on your Mac running OS X. If you become proficient at bash you can hold your own in the UNIX world. You also have access to editors such as Emacs and vi on the Mac, both come as part of the base install. Depending on the language you are programming in, you will most likely find the compiler already on the Mac (like gcc, g++, javac).
I like systems where I have everything already available. Today, installing packages is a breeze using Linux tools such as yum, apt-get, or up2date. In the "olden" days it used to be a nightmare - we had to solve dependencies, edit header files, and do a lot of praying. So maybe that is why I prefer systems where everything is ready to go.
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