Yes, the GMATPrep tests from GMAC are worth the higher price tag.
MGMAT tests are the best imitations, and do have certain advantages such as better timing and question classification information. But now that there are 6 full GMATPrep tests, I would agree with
dabral and others that non-official materials are not necessary for most GMAT test-takers. Plus, the
MGMAT scores are not as reliable as are scores from the GMATPrep tests, because
Manhattan GMAT does not have access to the proprietary GMAC algorithm, or real test questions for that matter.
Pro tip: to get timing information on your GMATPrep tests, simply train yourself to take a screenshot at the beginning of each question. For question classification information, use the GMAT Club question forums (a simple search for the text of your question should do the trick) and browse for Tags.
Why You Should Use Only Real GMAT QuestionsHow to access all 1,500 questions from your Free GMATPrep Tests