Where you end up living in the DC area will probably be the biggest factor, but I'd avoid IAD if at all possible. Security is a nightmare and DCA is so much easier to get to if you live near the city. I fly UA 95% of the time and usually leave out of DCA connecting in ORD. US Air has you covered for any east coast travel. I'll make the trek up to IAD on the rare occasion if it allows me to fly UA.