Yes a taxi will get you there more quickly, you won't have to find your way to the Air & Space Museum downtown and wait to catch the bus. Fare? Maybe $50-$60 each way.

How about riding Metro to DCA and renting a car for the morning? Weekend rates are cheap, I see as rates low as $19 on travelocity for Sat. There is a $10 parking charge at Udvar Hazy.