While DCA is the better choice, if you end up in IAD check with Priceline for a rental, we have done it many times down there for $17 a day, fly in on a Saturday, pick up the car, do what we had to do (the hotel we usually use doesn't charge for parking), get the car back within the day, and it's way less then getting to the city and back. Even when we are there for two nights, the car's sole purpose is to get us to the airport and back, never touch it.

It' one area I think Priceline shines in.