this is just my point of view.. but if you claim you want to learn unix what you are really saying is you neither know unix nor linux since they are both quite close.. but different in their own way! I apologize for the misunderstanding but unix and linux is used loosely sometimes since at our level (newebies and some) it doesn't make much difference what that difference is.. but I myself thought the same thing when I started but linux is actually a lot more advanced and developed quoting Paul Sheer author of Rute Tuturial and Exposition for LInux system admin.. he also tells that it is easier to learn. if you are downloading for a server type enviroment I would go with the "Unix" if you are going with the learning which is what is important then I would say as would many that linux is much more flexible, easier to learn with and more advanced in many ways and to go with linux then move to "Unix" .. I have taken a look at both.. and in one year I have learnt quite a bit and had to swallow my pride a little in many ways.. since I underestimated the power of linux, the difficulty of Unix and the true facts behind the two including the fact they go back to 1969.. i truely believe that if you are trying to learn unix the smarter choice would be to learn linux first unless there is some reason that you must use Unix like a Unix server enviroment.. even though if you know nothing I would still try out linux and read up on unix before making the move.. just my thought.
moxxx68