Just curious: does it really matter? Isn't which is more convenient the only thing that matters? I happen to like DCA a lot more than IAD, but that's not why I use DCA; I fly into DCA because of the convenience of the airlines' schedules and the proximity of DCA to DC. They could convert IAD into the Taj Mahal and that would not change my flying habits (except, perhaps, once so I could check the place out!).