Originally Posted by gumbotime

If you already have UA miles, then it might make sense to choose UA if everything else is equal, since the flight would get you a few more UA miles. Not a lot, it's only 629 miles round trip, but better to have more miles in a program you use than a few miles in a program that you don't.

Whether to get a rental car or not depends on where you're going in DC, but if you're going places that are accessible by metro, than not only are you saving on the cost of the rental, but you're saving a pretty significant chunk of money in parking fees if you're staying downtown.