I would pick Sociology, if you are on the fence. There are plenty of bio majors out there. In the current med admissions game, you need to do anything to make yourself stand out, and being a bio major won't help. So I say pick Sociology. You'll get plenty of bio in med school.
if you know for SURE that you're going to med school, then pick a major that will let you get goooooooood grades and will provide you with eternal interest. if sociology has these traits, go for sociology.