I concur with the Westin Embassy Row for an all-around convenient choice. The area is one of the more safe within the DC limits. There are tons of shops and restaurants around. The subway is 2 blocks away.
The Westin Grand is nicer, but it's a walk to any subway.
The Four Points is "decent", I wouldn't rate it higher than that. Plus, there's a certain "element" around that area after dark. It depends on how late you will head back to the hotel. It's not my favorite area to walk alone at night.
One other convenient location - Sheraton Crystal City. It's a decent hotel, there are many good restaurants within walking distance, the metro is right around the corner, literally. Plus, the ride in to the Smithsonian area is very direct. You also get a great view of all of the monuments by the water as you come over the bridge on the metro.
Have fun!