While DCA is closer to Union Station (for the transfer to Baltimore) You stated that the IAD flight is nonstop. You need only take a bus from IAD to the Metro and then transfer once to get to Union Station. From DCA you would skip the bus portion but would still have to transfer on Metro. Check out www.WMATA.com for complete details.

Do not take Slamtrak to Baltimore. Last time I checked it was over 50 bucks! Take the MARC (Baltimore commuter) train if at all possible for about 5 bucks. Some Slamtrak trains accept MARC tickets so that you might sit next to some poor guy paying 10 times as much as you. Hey, just like on UA.

Check out http://www.mtamaryland.com/