I have the 12th edition but I see it does not have an Integrated Reasoning section? Is IR on the GMAT test now and do I have to buy the 13th edition to study this?
I am planning to take GMAT in July same as you and after reading intensively on these forums there is no doubt that you have to pore over OG13 for the best prep since its the latest OG. Also if you buy OG13 you will get 50 IR questions to practice online. SO I think you should definitely look to buy OG13.