I say continue with the fifth edition. I have both, and am constantly going back to the early edition because some things are just better explained, like UBOs and PBOs (the latter is hardly mentioned at the end of the book in the 6th ed). Explanations in the newest edition are pretty terse. Also, the reference is cut out in the 6th ed; I guess it was to save some trees. I hardly ever used the reference, but it was a nice addition. Stick with the 5th version, and when you're ready to move onto atomic counters and compute shaders, the 6th version will be ready for you. Also have a look at g-trunc OpenGL reviews, if you haven't already. It'll put things into perspective of where OpenGL was back in the day, and where it is heading - especially when it come to the bind to edit model. ARB_bindless_texture anyone?
Edited by WhiteChocolateMocha, 02 November 2013 - 09:26 PM.