In my experience, pepto really only works if you been overeatting. As someone who lives with a spring, has many friends on springs/wells, and went to a college that is on it's own well, I've gotten to know the "joys" of the microfauna wars moving from water supply to water supply. Pepto never did diddly in those cases, but Immodium does.

Also, for the bulk, while pepto might work for heart burn and indigestion, if you need a long course because of the runs, Immodium packs smaller, lighter and has a longer shelf life. A two day course of the green stuff is about the same size as four or five matches.