The choice of course depends on what your goal is. Biochem is technically a topic on the MCAT, but the level of biochem/genetic tested on the MCAT is pathetic. Honestly you will learn much much more info in an average 1 semster undergrad biochem/genetics class than the MCAT could ever hope to test.
On the flip side, as stated previously, pathophysiology is a much more intersting (to me anyway) and thinking oriented subject. In addition, an intro to pathophys now will help you in med school (not much, but it might make your first couple weeks of the class easier). However, the same can be said of biochem/genetics.
From all my friends who went though med school with me (at various school inthe U.S.) most students are unhappy with the way their school teaches biochem/genetics. They are very dry, fact memorizing intense topics so understanding and grasping of key concepts doesn't come up much. There isn't too much difference between an undergrad or med school biochem/genetics class. You'll just be responsible for a lot more of the details and minutiae in med school.
Bottom line if you are looking for something to help your mcat studies, go biochem/gentics. If you don't really care and want a more critical thinking oriented class, go pathphys. Neither class is required at most U.S. med schools and only Biochem/genetics will be of any help on the mcat and even then, the amount on the mcat will be paltry. Good luck!