It depends. If you know you're going to be taking a taxi, or a shuttle, or a rental car there's not that much difference. If you are planning to take public transportation, Dulles is much more of a pain -- a bus to the metro. Whereas R-National's a straight shot to downtown via metro, which means no stairs to carry your luggage up, no folding your stroller if you're using one, etc . . .
Having said that, I almost always fly into BWI -- fares are usually much cheaper than Dulles or National (many of us in DC refuse to call it by it's "other" name), and it's definitely easier to get to than Dulles via commuter rail. Often times people don't think to check BWI because it's in "Baltimore" but in reality it's 1/2 way between the two cities. Have you checked out BWI?