I think their similar. Can't be too many diff ways to study for that test. Oh yeah, Kaplan's questions are tougher. I bought both books (Kap and PR) and except for teh questions, they were pretty similar.
I like PR's strategies a lot better, especially for verbal. They are much more helpful with figuring out the test. I would definitely use them, and also practice with the OG.
Hope I don't sound liek I'm bragging but I'm proud to say I self prepared and got a 720 this past week. I studied hard and got help from 2 books: ETS and Kaplan.
I think both books are good. They have a slight difference in strategies. However, questions are very different. KP question are more difficult than PR's. When did PR I got 710, and only 620 on KP. :} The more practice the better. I think it is a good idea to work through the tests in all possible GMAT additions.
I took the Princeton Review Class and it was helpful because it gave me a solid frame work to study and a clear plan with lots of test tips. It's much easier when you work methodically through a program than just buying a book and trying to study (at least for me). The actual class was just ok, the teacher was very smart but there were other students who really slowed the class down with poor or repetitive questions. My best advice would be to take as many of the on-line practive tests as possible and study hard. I scored a 710 but I only took the test once so there's no way to compare the value of a $1200 course to solo studying. If I get into a decent school I am sure it will be worth it.
I think the one common positive is the ability to get practice tests. Ultimately it's you that has to figure out what you really need to do to prepare.