Pete, the Courtyard Embassy Row isn't in a bad location, but parking is a bear and traffic can be difficult (which are always issues down in the District).
My other Marriott stays in the DCA/IAD area have been mostly in the suburban properties, but if you plan to use Metro trains to get around (which I recommend), that might be an option worth considering. Even if you have to drive 5 miles to get to your nearest Metro station, that could save you a lot of aggravation when driving and parking (ha!) in the District.
Wasn't there a property in Crystal City that was well spoken of on this board recently?