Depends on what level you are at.Kaplan
is harder. OG is easier than the real thing.
You should be OK working on either but both would be better. You are correct about OG being closer to the real test, but it is not as critical in Math as it is in Verbal. Math is much easier to match - verbal is a whole different story.
If you must choose, I would take Kaplan
, but that's me. (I never used OG for my prep) - did not know it existed
Founder of GMAT Club
Just starting out with GMAT? Start here... | Want to know your GMAT Score? Try GMAT Score Estimator
Need GMAT Book Recommendations? Best GMAT Books
Co-author of the GMAT Club tests
Have a blog? Feature it on GMAT Club!
Get the best GMAT Prep Resources with GMAT Club Premium Membership