GMAT Prep 1 Diagnostic: 680 (46Q, 37V)
MGMAT CAT 1: 680 (47, 35)
MGMAT CAT 2: 700 (44, 40)
MGMAT CAT 3: 680 (47, 36)
MGMAT CAT 4: 720 (46, 42)
MGMAT CAT 5: 740 (47, 44)
GMAT Prep 2: 770 (50, 44)
I think
MGMAT CATs are harder both in verbal and math. As you can see, I never got past 47 in
MGMAT Math, but got 50 on my most recent GMAT Prep. Also, I took each of the exams above about a week apart. So by
MGMAT CAT 3 (several weeks into my study), I started to panic b/c my scores weren't improving. But I just kept reviewing weak areas, reinforcing strong areas, and just kept studying, and my scores have been improving. I haven't taken any other prep providers' tests so I can't comment on which is best, but
MGMAT is pretty good. Their explanations of answers are very detailed and you can learn a lot by reviewing them. Also, obviously the books are very important. There have been concepts that I encountered on exams that I had reviewed before but forgot, so I go back to the readings to improve and review.
GMAT Club tests also have been helpful in practicing some more tricky problems.