GMATprep is by far the best predictor of actual test performance. Unfortunately,
MGMAT is not such a good measurement tool, although majority of test takers find it good for practice.
With some experience (and with other posters experience) some future test performance could be estimated based on your results on OG questions. I wrote about that a while ago, if you want Q50 you have to obtain hit rate of 99% on the first 1/3 problems in OG, 95% on second 1/3 and 90% on the last. Actualy, I had Q48 and my OG hit rates were slightly lower than what I've described above.
However, one should be carefull with this estimations in the last six months, because it seems that GMAT is becoming more and more demanding. What was true a year ago might not be now.