I've stayed at L'Enfant Plaza a few times on business trips, when I had meetings there. Very convenient for the Mall and most government offices, but quite pricey. It's also getting older now, and unless it's had a recent refurb, there may be better options. Parking at L'Enfant Plaza was also expensive as I remember, and there are no 'street options'.
I too would stay in VA near a metro stop, and Rosslyn would be one of my choices. There's a couple of older hotels over in Bethesda, that are convenient, but they were headed down years ago, so couldn't recommend either. I would avoid ANYTHING in Prince George County MD because some of the n-hoods even near the beltway can be intimidating.