I would recommend the Crystal Gateway Marriott since it is directly accessible to a metro stop via the underground tunnels. This property is nicer than the Crystal City Marriott (despite being a lower category), and it is the closest to DC. The Fairview Park Marriott runs a shuttle to the Dunn Loring metro stop on the orange line, and they have a nice concierge lounge. However, they have recently stopped giving free breakfast on the weekends to gold and platinum members.

I have not stayed at the Bethesda North Marriott Hotel & Conference Center, but it is within walking distance of the White Flint metro stop. Two factors are what airport are you flying into and whether you will have or need a car. Parking is free at the Fairview Park Marriott, but you will have to pay at the other two properties that I mentioned (as well as at the Key Bridge Marriott and the Wardman Park Marriott). If you interested primarily in exploring DC and Alexandria, you will not need a car. A car is useful if you want to visit Mount Vernon, take a side trip to Baltimore or Annapolis, or travel outside of the inner area around DC.