I'm in the same boat and always fly UA. Being less than 5 minutes away from DCA I usually do DCA-EWR-XXX most of the time for domestic and intl flight as it is much more convenient but on some occasions I take the drive out to IAD for some transcon and intl flight. Advantage of IAD is the use of LH lounge.
I do fly on a number of segments DCA-MCO on US which will have to be rerouted through EWR which has uauslly been a $200-$300 fare difference compared to US.