I live in the Dupont Circle area and so DCA is my preferred airport. It is also a much nicer airport than IAD for UA travelers. I do use IAD for long-haul international and some trips to the west coast--for example, if I need to depart California mid-afternoon, the only alternative is a non-stop to IAD. I have taken up American as a secondary airline for routes such as Washington-Boston or to Providence or Manchester/Boston -- going to these places and connecting in EWR is not appealing.

So DCA > IAD because of my geographical location and also the IAD experience is inferior to DCA, if typically flying United and using concourses C/D.