Rich is absolutely right - it's essential to study with the Official Guides, because that is the only resource of *real* GMAT questions. Any prep company's guides - no matter how meticulously researched - will just be approximations.
That said, you have to make sure that you're learning the principles and strategies behind each topic, so the strategy guides from MGMAT (or another company) are necessary as well.
I also highly recommend that you use the MGMAT Navigator to track the work that you do from the OG's:
https://www.manhattanprep.com/gmat/store ... t-tracker/
You can browse questions by topic and track your analytics.
Good luck!