Neither one will be comprehensive of all MCAT material (especially when you get a physical sciences section that is intensive on the properties of elements and metal alloys ) and buying both is a lot of expense. I just chose one (happened to be Kaplan), used that intensively, and then went to the local bookstore and the library to glance through Princeton Review and see if I had missed anything. That dinky local library has more than you might realize and can be very useful when you don’t want to buy a book. At the very least, I’d say study from one and then take some different free practice tests from other sources and see if there are topics you need to look up.