JM- Both airports have their advatages. I would probably base my decision on time of day, day of week, and where I was going in DC.

BWI is connected to Union Station by Amtrak and the commuter rail which runs on the same tracks. While the former is overpriced, the latter is a pretty good deal (less than $5, IIRC). The commuter trains don't run much, if at all, on weekends and I think you're out of luck if your flight arrives too late in the evening.

Washington Flyer buses will get from IAD to NOVA or west DC Metro Stations/hotels for a bit more than $10. (I apologise for all the vagueries here; I'm working from memory.) Washington Flyers run much more frequently than the BWI trains, but they are infinitely more susceptible to traffic problems.

Hope that helps.