If you can fly into DCA you are much better off, The public transport from IAD isnt good at all and it is also a fair ways out of town.
If we get a good rate we (bmi) try and put folks that come over into Jurys. It is a rather good chain.
To get to Jurys (Dupont circle) take the train fron DCA to Gallery plaza-chinatown (Yellow line) then change to the red to dupont circle heading towards shady grove.
Anymore info just ask as i live here