The two editions will likely be very similar. A friend of mine is expected to receive the 12th one of these days and I will update you once I've taken a look at it.
Here is what I already know:
The OG12 has 840 pages (
https://tinyurl.com/c8txjw) while the current OG11 has 832 pages.
There are about new 300 questions in the OG12. Since it is essentially the same size as the OG11, I would conclude that most of the questions will be the same (the OG 11 has 907 questions, so perhaps 1/3 of them have been replaced)
Argument for getting the OG12 right away
You will likely get a greater number of difficult questions because that has been the GMAT trend over the past couple of years (average GMAT score has gone from 530 to closer to 540 I think).
You'll get to show off your new book to the OG11 crew and feel good about yourself.
hmm... honestly I don't know why else you would get it. There hasn't been any major change in the content tested.
Argument against getting the OG12 right away
The OG11 has been the standard of all GMAT prep programs for 5 years now; by keeping the OG11, you can make better use of the other materials that have been designed to work with it. I'm thinking in particular of the MGMAT guides that assign homework in the OG11 and of The Official Guide Companion from gmatfix; If you use these or similar guides to learn the content, it only makes sense to keep the OG11 so you will know which specific questions you should work on to practice a given topic.
If you switch to the OG12 right away, you may learn the content elsewhere (the OG was never designed to teach, but only to offer practice) but once you've finished the lesson on, say "Inequalities", how will you quickly determine which 60 questions out of the 900 would allow you to practice and perfect the lesson? If you have The Official Guide Companion from gmatfix or a strategy guide from ManhattanGMAT, you would just go to the reference section and identify where to go in the OG11.
Basically, think of the OG12 as Windows Vista when it first came out. It was new and shiny, but the software around was mostly designed to work with Windows XP, so although the Vista users got a big boost of pride, they just couldn't do a lot of things that XP users could do. Vista has become better as the market has adapted.
