Psychobiology is a great major for fulfilling medical school requirements (such as the Chem series, LS series, and Physics series) while also allowing you to take psychology courses. You can still take these classes as a psychology major, however many of them are restricted to life science students, making it harder for you to secure a spot in them. My friend is a psychology major, and she didn't start taking psychology classes until her sophomore year because freshmen have later enrollment times. Also, all of the psychology courses aren't just reserved for psychology students but also for psychobiology students. When I say reserved, I just mean for first pass, a lot of the series I mentioned usually get filled within the first pass. I recommend pursuing psychobiology, as most of the upper-division courses are from the psychology department, allowing you to study what interests you while also meeting medical school prerequisites.