Hi there, have only done 2 MGMATs and 1 GMATPrep, while gone through all "official" prep material and MGMAT SC guide, so take my advice with a pinch of salt:

Although I agree with most people who point out that MGMAT tests are the closest to the real thing, I´m not quite sure about MGMAT Quant being much harder in MGMAT than in the actual GMAT. From my point of view, Quant in the MGMAT is just different, not harder, it focuses far more in combinatronics and it´s problems are a little longer, while focusing less in number properties and probablity, compared to the GMAT.

What I do think, on the other hand, and haven´t seen it mentioned elsewhere, is that Verbal is really more difficult in MGMAT than in the real thing. Verbal has been, almost from the beggining, my strong point in the test (I said verbal, not writing hehe). I´ve had >90% accuracy in both RC and CR, while achieving around 75% in SC (the weakest part of my game). In my only GMATPrep test to date, I scored 41 in verbal, and when I reviewed the questions my right/wrong questions were pretty much in line with my assesment (RC>CR>SC ). After my 2 MGMAT CATs I have struggled to go further than 36 in V (I had two IDENTICAL scores on my two exams), while doing much better in Q. Furthermore, albeit being mediocre in SC (which is not surprising, hehe), I really underperformed in RC, which leads my to believe it´s not really the same difficulty level as in the real GMAT (or maybe I just want to believe that).

Well, those were my 2 cents

Cheers