As would an MPH. Also an MPH would strengthen your application for residencies, who will not care in any way about your undergraduate research experience.Research would strengthen your application for the med school application process.
this. i suspect the reason OP is torn over this is because s/he doesn't have enough background in either to make an informed choice. so, leave both options open as long as possible until the preference becomes clearer. take classes for the MPH; if it isn't right, get out.As would an MPH. Also an MPH would strengthen your application for residencies, who will not care in any way about your undergraduate research experience.
OP, my advice is not to structure your life around what looks best to medical schols. If you have an interest in epidemilogy, infectious diseases, or healthcare management get an MPH. If you think you might one day want to be a researcher do research. If you don't want either and are just trying to game the game do neither and do something you are interested in instead.