I believe the best way to study for the MCAT is to use either Kaplan's MCAT material or the Princeton Review. Both have strong and weak points.
Kaplan's topical tests are in my opinion much better than the Princeton Review's but they do seem to be somewhat harder than the actual MCAT test. That may or may not be helpful to you depending on the way you study.
I've found the Princeton Review seems to match the MCAT in difficulty much more closely but are not as concise and don't have nearly as good of a physics section as the Kaplan tests do.
Don't be overly concerned if you get around a 13 on your Kaplan Diagnostic practice test---it's perfectly average and just keep at it. So yeah, I'd say stick with Kaplan more than Princeton Review---their tests are hard but prepare you really well for the actual MCAT.
Hope this helps!
