I don't think there's any overlap between PowerPrep and GMATPrep. PowerPrep is now quite old; it was only available on CD (not by download), and contained two simulation GMAT tests, but with a relatively small database of questions. At least some, perhaps many, of the PowerPrep questions have been reprinted in the Official Guide. If I recall correctly, GMATPrep was introduced when the test producer changed from ETS to ACT. GMATPrep is a far more accurate representation of the current GMAT in all respects. PowerPrep's probably worth doing if you can find it and you have a lot of time to study, but don't worry about it too much- GMATPrep is much, much more important.