From my personal experience, and I live right near princeton, lol.. the Kaplan review is better for the sole reason of being better structured? Not sure if that's the right word.
Basically, anyone can teach the princeton review as long as they score a high enough score on the test themselves, they don't have to be good teachers. So sometimes you get great teachers, othre times they blow.
With Kaplan, you have to score well, then udergo pretty intese training, and they make you teach lessons and evaluate your teaching style before letting you teach a full class. And maybe its your area, but I had about8 people in my Kaplan class, and have heard about huge princeton reviews, so not sure about that.
Finally, it must have worked somewhat b/c I managed to get into grad school for my PhD (I took GREs) And I think I def. learned some things, and the flash cards helped a ton.