Okay, I just finished the AAMC exam, and I'm really happy with how I did compared to the Kaplan FLs. I am just going to compare Kaplan FL2 and the AAMC test because I took them three days apart in the same testing environment. As a disclaimer, I did study general chemistry (my weakest subject) and a little psych during those three days, so some score improvement may be attributed to studying.
Score comparisons by percentage of questions answered correctly:
Kaplan FL2: Chem/Phys: 39% CARS: 88% Bio: 76% Psych/Soc: 49% All Sections: 67%
AAMC: Chem/Phys: 69% CARS: 98% Bio: 71% Psych/Soc: 88% All Sections: 81%
My overall impressions (n=1):
Chem/Phys: In general, I think AAMC was even more bio-based than Kaplan (which I liked). I thought the AAMC test had more biochem, less physics, and less gen chem than Kaplan. I thought AAMC had a little more organic than Kaplan, but most of the organic questions were incorporated into biochem passages.
CARS: The two tests were fairly similar. I enjoyed the AAMC passages more, but that will vary between different tests.
Bio: Similar content covered.
Psych/Soc: This is where I saw the most discrepancy between the two exams. My psych studying between the two tests was minimal (1 chapter). I think the huge difference in my Psych/Soc scores was due to significant differences in the exams. There seemed to be fewer questions on specific scientists/theories on AAMC than Kaplan (although AAMC still had some of these questions). AAMC seemed to have more application-based questions and some common-sense questions.
Please keep in mind that I am just one person and these are my subjective impressions. If anyone else has taken both of these exams, I'd love to hear your comments!