SanFF, I don't mean to scare you, either, but blueDC has it right... traffic around here can be HORRENDOUS due to many factors: congestion, bad signage, construction, people who don't know how to drive...heck, these days we even have to be on the lookout for "dive-bombing" cicadas! (bugs) And if the weather sucks, fuggedaboutit.... people totally wig out here if there's so much as one drop of rain. So if you're going to drive, you do need to allow yourself enough of a time cushion. Also, have you checked the IAD transportation page to see all your options? http://www.metwashairports.com/Dulles/ground.htm

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