while preparing for Gmat, I ve used veritas, manhattan prep, and experts global mock test series beside GMAC official tests
Based on my personal experience, I would say experts' global mocks have a little edge over other
Pros The scoring algorithm is good ( after giving 4 mocks, my official score matched the avg. of veritas mocks), the test is adaptable
Cons: RCs are tough and 3/4 are long RC, which is hardly the case in the actual exam.
Pros Verbal section is solid, full proof, and a little tough but score matches the official score.
Cons: Quant is quite hard, for instance, if you are getting 44-46 in Manhatten mock than you will achieve 49 in the official test
Pros: Sections difficulty is quite balanced, the scoring algorithm is good, answers explanations are comprehensive, and the most important question structure is quite Gmat centered.
Cons: on some tests, I've fount quant little on the upper side of difficulty but it might be because of test adaptability.
I hope the above comparison will help you with choosing the right one