i'm also wondering the same and would like to know other people's thoughts. I've done mostly
mgmat CAT tests and I've noticed the math is more difficult than that of the gmat prep and easily more difficult than OG or 2nd edition quant review (i'm done with all the OG material now). I've seen at least half a dozen people echo the same sentiment and even add that it is more difficult than the real thing.
however, some people also say that it's a different kind of difficulty. the
mgmat problems often involve more steps to solve than the real gmat problems and therefore take more time. although the real gmat problems apparently do not take as much calculation as the
mgmat problems, they are more difficult in that they have a really elegant trick to them. i always thought it was difficult to write very good quantitative problems so perhaps manhattan just makes up for that difficulty by making the problem more convoluted with calculation, wordiness, or just tricky wording. for that reason I'm shying away from using their material.
anyway, i asked on another thread about the samurai problem collections. there is a collection of DS and a PS and it contains maybe a dozen questions per topic. Bunuel mentions they are 650-750 level. This is free so this is something I'm considering.