Well tartrate, most research/studies show that studying at the same time and place everyday is more effective than changing places due to associative learning and cellular/body memory which allows your body to get into the "right state of mind", allowing you to be more ready to learn. Obviously, you want to be as uniform as you can so you want to study frequently, same location, same time, wake up/sleep the same time everyday, etc so your body can get used to your schedule. If you do the same for the MCATs and realize the afternoon is when your body is telling you it is energized and focus because you're just not a morning person, then you take the afternoon MCAT. If your body tells you do better on practice tests in the morning, then take the morning MCAT. Studying efficiently and test taking are obviously skills one develops over a lifetime so just by look at your own sleep schedule, test performance, level of alertness, etc you should be able to tell which test is right for you.
Personally, since all the SATs, SAT IIs, and other standardized tests in high school were in the morning, I am very used to the morning test and feel as though I would kill myself if I had to wait around for the afternoon test.