I actually found the Kaplan tests to be harder than the AAMC tests. If you do sign up for kaplan, then you get all the AAMC tests offered.
My thoughts on Kaplan, they have great materials and good instructors. They genuinely want you to do well. I liked the class, and do not really regret dropping the $1500; however I needed the class more as I had been out of college for 7 years and was rusty on a few things. If you are up to doing your studying all on your own, that can be done too. You can usually buy the kaplan books on craigslist for really cheap, and Kaplan, Princeton, examkrackers can all be found on the internet