I have found that the OG questions are too easy, and that the MGMAT questions are extremely hard. Is the real GMAT somewhere in between? Anyone who has already taken the test could shed some light on this subject.
OG is a waste because its too easy.
MGMAT rocks because it's pretty hard.
My take is that to do well on this test, you need to be very comfortable with the hard stuff out there so my vote totally goes for MGMAT. As mentioned above, just be aware that MGMAT is quite hard. Learn from it and don't let it shatter your confidence.