An Intro Psychology class will most likely also cover the basic parts of Sociology that you would ever be expected to know for the MCAT (via a discussion on Social Psychology). While both are probably adequately covered within review resources, I think that Psych is a better class overall and is probably the more "difficult" of the two topics, so reinforced learning might be best. Psych will also talk a lot more about research design (hopefully) compared to Sociology, which is *essential* for MCAT prep nonsense.