Great question. There aren't that many differences between the two; you can see a write-up of the differences here:
http://www.manhattangmat.com/official-guide-12.cfmhttp://www.manhattangmat.com/ogs2-verbal.cfmThere are only 300 new questions in OG12; the other 600 were already in OG11. And there are only about 75 new questions in the new verbal supplement compared to the old one. If you find that you need some new study material, then these are great questions to do, but you don't need to feel like you have to get these. Totally up to you. :)