There are so many myths floating around as to comparative quality of the Kaplan or Princeton Review MCAT course. For instance, at UT Austin the vast majority of pre-meds erroneously believe that PR is better, even though Kaplan is the superior program. The fundamental problem is that very few people take BOTH programs in their respective areas. I've only met one person in Austin who has taken both Kaplan and PR; he believes that the Kaplan program is superior to the PR in Austin.
My point is that you shouldn't base your decision solely on hearsay. Go to each place and talk to the director (or, ideally, a senior MCAT instructor) to learn about the program. In particular, you ought to find out the following:
1. Where are the full-length practice tests typically held? (Ideally, they should be in the same place that you'll take the real MCAT)
2. What is the philosophy of Kaplan/PR regarding MCAT preparation? As an instructor at Kaplan, I can tell you that our philosophy is to integrate critical thinking, content review, and crisis prevention throughout the course. In addition, we don't overload students with scientific knowledge; rather, we teach the material that crops up very frequently on the MCAT. Instead of inundating students with facts, we prefer to emphasize test-taking strategies. Make sure that the program's philosophy dovetails with your needs. Kaplan isn't for every pre-med--the same goes for PR.
3. What practice materials are available for MCAT students? How do students access them? Make sure that the program has all of the AAMC practice items (including the full-length exams), as these tests closely approximate the actual MCAT in terms of difficulty. Most importantly, make sure that access to the materials is compatible with your schedule and preferences.
4. Check out the classrooms and training areas. You'll spend an inordinate amount of time at the center, so make sure that you like the facilities.
5. What books and training materials are issued to students? Browse through these materials.
Good luck on the MCAT!