Originally Posted by Gator Gal
dlouise37, as to which is the best airport transportation wise, it's DCA hands-down. It's right on the Metro route (blue/yellow lines) and just 10 minutes from DC. You could take either train and transfer to a red line train to Union Station and then take the MARC (commuter train) and shuttle bus combo to BWI... see the WMATA (Metro) "Ride Guide" comment in greggwiggins's post above. Hope this helps!
ETA: Beltway traffic on Saturdays isn't usually too bad
If you can get the same fare into either DCA or IAD take the DCA routing because it'll be much,
Much, MUCH easier and faster to get to BWI from National than from Dulles. But keep in mind if you're traveling on a Saturday that the MARC commuter trains don't run on weekends and you'll need to travel AMTRAK from Union Station or the Metro rail/bus combo from Greenbelt.