I would recommend the 11th edition. Explanations are much better and so is the layout. For example, in the old edition the answers are embedded in the solution explanations. Whereas in the new edition, at the end of each section you have a summary page with all the answers (much less cumbersome when checking your answers).
The only reason I would recommend buying the OG10, is that it contains more problems. For the new edition, GMAC decided to provide more depth in there explanations and subsequently reduced the number of problems. However, they now offer two additional workbooks which contain all the questions from the OG10 which are not included in the OG11.
IF you don't care about the explanations and are primarily interested in the problems, then get OG10. Otherwise get the OG11 with the Quant. and Verbal workbooks.