Thus far in my postgraduate academic career I have leaned towards addressing my (US MD) medical school professors who are MDs, MD/PhDs, or PhDs as "Professor", since we are not in a clinical setting. Today two of my classmates addressed one such MD as "Dr." in an email chain following my reply, as if to correct me (though maybe it was out of habit).
Was I less respectful than I could have been by addressing the professor as "Professor"? The latter title seems more prestigious in the context, but maybe I am mistaken.