Both of these sound quite ugly, but your colleague’s suggestion might have some useful merits if we could say, for example, that “the essential difference between the two (public) classes is the particular set of features that they do or do not expose (said features being taken from a common bag).”   If this way of thinking about the problem proves useful, then it suggests that the common-ancestor class is never meant to be publicly exposed or used; it has no un-protected getters or setters of its own.   But it does provide “the bag of tricks” used in various ways by any of its descendents.   The ancestor provides the universe; the descendents provide the various accessible heavenly bodies.