Physiology for the MCAT, Biochem for med school (altho physiology is nice to have before med school as well).
I dont think I had any straight biochem questions on my MCAT. I had a few passages on straight up physiology. Could you have figured them out w/o having the course? Sure, thats why you buy MCAT review books that will cover what you need to know. But, it was A LOT easier b/c 1) i didnt have to study that portion of the bio test b/c i knew it already and 2) i knew it better than I would have if I studied via review book so the passages were pretty easy. I basically could tell what questions they were going to ask and had a good idea of what the answers would be.
I vote physiology for MCAT. But again, if you can't take it before the MCAT its not really a big deal.