I'll comment on what helps you to prepare for the MCAT.
Agree with this. Don't worry about med school content. Your success there will depend on how much you study during med school, not what you did before you got there.
Prep for the MCAT and take what ever interests you most after you've cleared that hurdle.
I disagree. Being topically familiar with a subject does help you out in med school a little, although you do end up learning something in about 5-10 times more detail than in undergrad.
In some subjects (neuroscience), detail is far greater in medical school. In others (physiology), detail has been about the same. With biochemistry, there are more facts to learn in medical school, but the depth of the understanding was greater in undergrad/grad school.
I agree with the others about not worrying about med school content until med school.