Normally I would say that it won't make a difference, but with the limited information that we have for the 2015 MCAT, the materials may improve once the actual test comes out. Some of the better materials (TBR and Examcrackers) don't have their material out yet, so it may not be worth it to buy the information now. I am taking the test in April, so my pickings are slim. I would wait until more material is out there. If you really want to get a start on content review, use the free Khan Academy resources. Everyone will tell you that doing content review too early is a waste of time.