GMATPrep is a much more accurate representation of the real GMAT, though there is some value to doing PowerPrep. The database of questions on PowerPrep is much smaller than on GMATPrep (hence more repeats), PowerPrep contains several questions you will have seen in the OG, and GMATPrep has a much larger pool of difficult questions, so is better suited to the high-level test-taker. In addition, GMATPrep simulates the current GMAT in all respects- appearance of the screens, style of question, etc. If you want a good predictor of your likely score, GMATPrep is a better option, and I do think there is a lot of value in doing each GMATPrep test more than once. The PowerPrep tests are worth doing once each if you have time, but should be a lower priority than GMATPrep.