It depends. A bio or chem major, at my school anyway, required quite a few additional upper level science courses (courses that weren't med-school pre-reqs). Also, some of your med school pre-reqs might not count towards your science major -- organic II, inorganic II, and physics I & II, in my case. Essentially, as a science major doing pre-med requirements, youll end up taking WAY more science classes than would be necessary to complete your major or your pre-reqs independently. I hope this make sense.
My advice, choose a major that you enjoy as the subjects that you like to study will likely be the ones in which you excel. I chose a bio major b/c it was what I enjoyed and where I excelled. My grades in the easier classes sucked b/c I wasnt as into them.
My advice: dont worry about easy vs hard STUDY WHAT YOU ENJOY!!