We've stayed at Crystal City Gateway Marriott two years ago. It was super convenient Metro access just a quick walk and lots of convenient shops nearby and you don't even need to go outside. We didn't have a car though, shuttled from DCA. What I like about staying closer in are the super convenient Metro access and all the shops. I wouldn't stay in areas that are end of the line Metro stops, ie Springfield in my opinion. Not that the places are bad, it's just that I would want more convenience and to be closer in if I came on a visit.
I've also stayed two weeks at the Residence Inn Merrifield while we were moving. It was far from the Dunn Loring Metro, I guess one could walk it but again, not super convenient if you come on a trip, but they did have a free shuttle service and you go ask the front desk when you want to go to the Metro, and called them from your cell at the Metro when you wanted to be picked up. Parking was free, and there was a little shopping center right next door for Starbucks, CVS, Aldi, etc. However, if going for vacationing I would stay further in, Arlington, Alexandria, Crystal City, Rosslyn etc. something that's just a few stops in from where you plan to Metro to and ease in walk to the station.