Very surprising as I have quite literally never seen the FS cheaper than the RCGT. If I may ask, what rates are you being quoted? The Ritz only has three room types--room, suite, and premier suite, so upgrades from room to suite are easy to negotiate and frequently possible.

Having said that, what are you looking in your hotel? The Ritz is a true boutique with only 86 rooms. The front desk staff double as concierges and are akin to personal assistants in my experience there. I've had everything from doctor's appointments to dry cleaning to elaborate dinners arranged by them, and always with a genuine smile and desire to assist.

The FS does have more functional spa and pool, as well as a true destination restaurant in Bourbon Steak. The F&B is also better overall. However, the FS has ~250 rooms and no real 'personal' touch in service, at least in my experience.

My preference would be to stay at the RCGT in a suite, and eat/drink/spa at the FS as it's less than a five minute walk away. If you do do so, request a north facing suite in the West Wing--you'll have a quaint view over Grace Church and the buildingtops of Georgetown.