It really does not matter, especially since either way it's only a quarter long course. Take what sounds more interesting to you, what you feel you'll enjoy learning the most, and don't do anything thinking it's going to give you a huge leg up on med school.
Personally I enjoyed genetics a lot more than biochemistry because it seemed more directly applicable to clinical knowledge at an undergrad level, but biochem is probably more likely to show up on the MCAT. Of course it would only be as background information and nothing in the course would be necessary to doing well on the exam. I hope this helps a bit and good luck no matter what you choose!