Linux and Unix are essentially the same thing, and for the purposes of beginning learning I would call them identical.
linux, however is free (as in beer) and unix is generally not so I'd recommend getting linux.
also it's my understanding that OSX is a cut-down/restricted/heavily customised version of unix, so you'd be better off starting with something like ubuntu (or even backtrack if you're going to use it purely for learning computer security stuff).
What languages are you thinking of learning first?
(my personal recommendations are: C/C++ followed by Python and Perl, then brush up on SQL and database interaction if you don't already know it.)
I learned VB, but found it to be very very limiting, so when I hit it's limit's I stopped learning it.
Thank you for the advice, I wasnt sure what language I was going to learn first. I have an Iphone, and I have a Terminal application on it. I have started learning some basic Unix commands, and found my little Iphone can do a lot of really big stuff

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I will take your advice and start learning C/C++ I'm pretty sure sure Iphone apps use C, or at least a version of C. So that would kill two birds with one stone for me.
My main goal is to get better at at computer security stuff, so I will try to find a VmWare image of backtrack.
Thanks again for your advice to my noobieness type questions