Parts of Arlington county might also make sense to stay at as well (despite easy highway access). Crystal City, you can travel to/from on the metro and walk underground without going above ground once you are checked in, from a few hotels. Also, lots of Pentagon- and DOD-related cameras and resources... to say the least in that area.

Rosslyn or Courthouse area of Northern Virginia may make sense too, because the roads around there can be "choked" off easily and quickly in all directions (despite multiple roadways/highway intersections)... between Pentagon-related security on one side and the Arlington County police and sherrifs on the other based in the Courthouse-metro area. There are a lot of traffic cameras in the area too, so the sniper(s) time would be up if he struck in those exact areas.