As much as I love Princeton Review and recommend it to everyone who is considering taking an MCAT course, I have to say that the one aspect that Kaplan is better in is their online material. They provide you with a TON of content, subject quizzes, topical tests, workshops, recap of class lectures, a Q-Bank, and many full-lengths. In comparison, Princeton Review has only 10 of their own full-lengths, all the AAMC practice exams, and a few quizzes.
Despite that, I still think Princeton Review has better and more detailed books and I like how their course is structured.
I'd go with Princeton Review.