I found that with the Kaplan tests the PS and BS sections were harder. To compensate this fact, they made the curve much more generous (usually you could get 6-8 wrong and still obtain a 14). The AAMC tests are much easier but the curve is harder.
I'm not sure they transfer, but I felt that the Kaplan tests were slightly inflated. My AAMC average was 36, my kaplan average was around a 38. And for my actual exam... well I won't know how I did until the 13th of october.