Here are some words in favor of the Bethesda Hyatt---it's surrounded by great dining and sits right on top of a metro station. Location is very convenient--subway downtown is about 20 minutes-max.
I've had lots of friends and family stay here, no one has had serious complaints.
Of course, if you've got someone else willing to pay big bucks for your room, go ahead and book downtown. The Park Hyatt houses Melrose, one of the finest dining rooms in DC.
If you're working in Tysons or Reston, book out there.
If you're on a budget, or would like a quiet location where you can walk around and restaurant windowshop, stay in bethesda.