The official guide is published by the people that give the actual exam. The questions in the OG are the "real thing".
I've looked at questions from other providers, like Kaplan
, Princeton Review
& ARCO, and I can say with certainty many of the questions appearing in these books bear no relation real GMAT questions. They have flawed logic and test obscure tricks will not help you on the GMAT. These questions could be easier, or harder, but will not help you prepare because they are so different than real GMAT questions.