Based upon the varied postings here, there is clearly not a right answer to this question: ie. neither Kaplan nor PR is "better." I can tell you about why I chose Kaplan and why it worked for me, though. At the same time, I'll let you know its drawbacks.
I am a non-trad, having graduated from college in 1999. Consequently, I haven't taken gen chem (for example), since 1995 or 1996. Taking some prep course was absolutely essential. When trying to decide which, I asked around. A friend of mine had taught for Princeton Review and he initially sold me on them based upon the rationale that PR really vets the teaching skills of its hires. According to him, at least, Kaplan primarily cares about how its teachers scored on the MCAT, not how well they teach.
However, when I discussed the matter with my pre-med advisor, he offered some counterpoints. (Warning: I have not checked the following facts. They are hearsay.) First, Kaplan has been in the business longer and has standardized its program nationally. As such, every Kaplan center offers essentially the exact same program of study. Conversely, the quality of the Princeton Review varies from center to center. Second, and most important to me, Kaplan requires about
half the number of in-class hours that Princeton Review requires. (I did check up on this.) That was essential to me, as I work full time, and I generally do a lot of my learning on my own anyway. In fact, that sealed the deal for me, in terms of my decision.
As for my Kaplan experience: it was mixed. My class only had about 6 people in it, and I took it in downtown SF, so the classes are not necessarily going to be huge. (I also took the one that started in the fall that only meets once a week, so that may have had something to do with it.) As far as results go, I couldn't be happier. I kicked butt on the MCATs, raising my initial diagnostic by about 17 points. How much of that was due to Kaplan, I can't honestly say. Certainly, their materials were helpful, and the structure and discipline they provided were essential, but I found their "system" to be a load of BS...for me anyway.
A couple of other important points, if you decide to go with Kaplan:
---The quality of the teaching varies tremendously. My teacher was absolutely fantastic, but I sat in on another lecture when I had missed one of my own, and the guy was horrendous. Dead man lecturing. If you feel like you are going to rely on the classtime to learn the material and your teacher is bad, I would advise you to look elsewhere if you have options.
---Their practice tests are good for practicing how to sit there and concentrate for 7 hours, but their content still doesn't quite reflect the actual MCAT. The practice MCATs created by AMCAS are MUCH better indicators of how you are doing with the material. I highly, highly, highly (did I mention highly) recommend sitting down and taking a full length practice AMCAS MCAT under test conditions at some point when the test is approaching. Looking back, it probably would have been a better use of my time to practice with AMCAS MCATs over the course of my studying in place of some of the myriad Kaplan practice and diagnostic tests (they have 10 million, more or less). Kaplan will tell you that you don't need to get any other materials to prepare for the MCAT, but I disagree. You should invest in the AMCAS practice MCATs.
More than anything else, I needed the materials, and I needed a structure which would encourage me to partake in a disciplined course of study. If that's primarily what you need, you probably can't go wrong with any of these courses. If you feel like you are going to be very dependent on the in-class teaching component of the course, be sure to thoroughly investigate the teachers. And, let me reiterate: I and others I've talked to, found the AMCAS MCAT practice test to be the best practice test out there. Don't forget about it.