Originally Posted by ReticulatingSplines
WMATA is only reliable in the sense that it reliably lets you down. Keep the car. Just last weekend they shut down several entire stations for track work, and buses frequently just don't show up. Mount Vernon by public transport is a long train ride plus a bus connection.
Or a cab from the Huntington Metro station.
A car would certainly be easier, but is it worth 125 extra dollars?
Metro is pretty reliable, weekend track work notwithstanding. Those can be a pain. Depending on what day(s) you'll be in DC and when you're going to want to go where.
One of the Smithsonian museums (the Air and Space Museum's Udvar-Hazy center) is located 20ish miles west of town; if you're interested in going there a car would be useful (though there is some kind of bus that runs - for a fee - from the A&S museum location in the city).