It is absolutely the case that GMATPrep is the most like the actual test. Unfortunately, you only get two "clean" tests; after that, you can see questions you've already seen before. Overall, it's a good idea to take about 4-5 tests throughout your study period.
The official GMAT has a standard deviation of about 30 points. GMATPrep's standard deviation hasn't been published, but it is probably close to but not quite as good (the algorithm is the same, but the question pool is substantially smaller), so let's call that 40 points.
Our (ManhattanGMAT's) practice tests have a standard deviation of about 50 points - not as good as the real thing, obviously, but close. (This is calculated by gathering our students' results on the official test and matching those results to the students' results on their individual practice tests.) I'm not aware of any other companies that have posted standard deviations for their tests - you could check their web sites or call to see.
Please note: I do not use the Private Messaging system! I will not see any PMs that you send to me!!
Stacey Koprince
GMAT Instructor
Director of Online Community
Manhattan GMAT
Contributor to Beat The GMAT!
Learn more about me