Manhattan Prep publishes new strategy guides every few years, to reflect ever-improving pedagogy. Any edition of our books would contain all of the content you need to study for the GMAT, as the content doesn't change (with the exception of IR, which was added in 2012, and which we've included since our 5th edition). We just like to update how we package / present / teach that content.
The biggest difference between our 5th and 6th editions is including specific chapters on strategy (Testing Cases, Working Backwards) in addition to embedding these strategies into the content-specific chapters. The content-specific chapters will be organized in much the same way as they were in the 5th ed. You certainly won't be missing any information - you'll just have additional chapters.
We no longer update our books with each new edition of the OG (we used to publish the OG question #s at the back of each guide), as GMAC will be publishing a new OG every year, and we publish new editions of our guides about every 3 years. So any edition of our books will fit any edition of the OG.
When in doubt, I would recommend buying the newest versions of each, unless you're following a Study Guide that references specific page numbers, etc. From what Brent said, I think you'll be fine using the 6th edition.
Good luck!
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
