Theoretically, D.O.'s historically were trained to treat the "whole" person while M.D.'s were about treating the symptoms/illness. In current history, training of M.D.'s has changed so that they, too, theoretically treat the "whole" person as well. I know my experiences with our personal M.D.'s has been that we have some really good M.D.'s who are very much this way although as in all things, no two doctors are alike. A D.O. is also trained in osteopathic manipulation, which I know nothing about, but many of them don't use the technique in their practice of medicine so it becomes a moot point with them unless they do.

Both degrees require the same training (with the exception of additional training in that osteopathic manipulation that I mentioned), have to pass the same licensing requirements, etc., so it isn't a matter of one being more qualified than the other.