Unless you have a specific reason to be in Maryland, I'd say camp in Virginia.
Bull Run Park (near the Manassas battlefield) is within a 15 minute drive of METRO and 35 minutes (non-rush hour) of the mall. Even though you are in the middle of a built-up metropolis, the sites are large, privacy is excellent, and the rates are cheap. We stayed there for nine nights last year and we found it to be quiet; I can't comment on how it was during daylight hours as we were doing the tourist thing. Camp staff was probably the friendliest we have ever encountered, and since there is a major shopping area within 10 minutes drive, including 3 grocery stores, if you forget something it's no big deal. Their web site is here
. Bull Run Park is run by the same folks as Pohick Bay.