I guess it depends on what you want... if you want to study biological sciences, you'll have to go to grad school for biological sciences, but there's more to medicine than just that. Public health can cover a range of fields, from public policy to international health to epidemiology - the study of disease in populations.
What is it that you're looking for?
Also, if you're referring to flexibility of career path, an M.D. does provide a good deal of that... you can be in academics, private practice, research, public policy, etc. In fact, a PhD who I used to work for said that, if she could go back and do it again, she would have gotten an MD, because it would garner more respect, more flexibility, and higher pay, and she could be doing the exact same research.