It depends where exactly you are going. I always fly into DCA because it is better located for DC. (IAD is far away.) Also, DCA looks crappy but it's fast (security lines are very quick, luggage comes out quickly). IAD is bigger and I find it more of a mess. (You have int'l pax as well, people with more luggage, etc.) I don't have a good enough basis for comparing UG's.