I took the PR class, and I took two of their full lengths. I thought the physics and chem was way harder then the aamc practice tests (I took all of those), but I hadn't finished my content review by that point. I just took the actual MCAT this weekend, and my advice would be to use them for timing and to get a sense of some of the harder questions they might ask, but use the AAMCs to refine your timing, get a sense of some of the experimental passages, and gauge your score. For what its worth, my scores on the AAMCs were much higher then the PR. On my MCAT, there was a mix of questions that I thought was really unlike either set of exams: the passages were far more about being to understand what the passage was about and maybe apply a really general principle (like the AAMCs), while a lot of the discretes were about really specific minutia (like the PR).
And when people say to "try and replicate the test conditions", its good advice. Although I'm not sure how close you can really get without asking your local prison if you borrow a cell to take the practice tests in.