VERY DISSAPOINTING - WE BOOKED THROUGH HOTELS.COM WEBSITE.. DON'T DO IT
Unfortunately, unlike most other reviews our experience was very disappointing. From check in things did not go well, upon check in when looking to purchase an upgrade, the check in clerk made a most inappropriate comment. We got to our room which was not what we thought we booked (Be aware that a superior room is the basic room, The Deluxe room here is above superior !).
Upon seeking to speak with the 'Manager' of the evening to discuss a bigger/better room, as we were staying a week (and happy to pay), and to address the inappropriate comment upon check in, we were moved to another room which we were assured was bigger, it was not. At that time I again asked was it possible to purchase an upgrade to a bigger room as they appeared available on the Sofitel Washington website - I was again told this was not possible they were booked out !
The next day I sought to speak with the Guest Relations Manager to try and understand why were still unable to purchase an upgrade given varying websites stating that they were available for purchase, to explain our new room was not 'bigger and better' , and to confirm that the behavior/comment of the check in staff was not condoned. It was at this point in time that the Guest Relations Manager whilst very engaging and chatty explained that Hotels.com customers were the Hotels quote 'Nemesis'. 
You can imagine my disappointment and surprise! Management may think this is the case , but telling the customer !!!!!! This was clearly why we had been treated as '2nd' rate clients and not worthy of trying to resolve our concerns. Still it was never explained why we were unable to purchase an upgrade room. 
Whilst yes the hotel is convenient to the National Mall, and walking distance to all of the Smithsonian Museums, we found it quite pretentious and clearly not accommodating when identifying any issues. 
Out of the 8 hotels we have stayed in the USA over the past 2 months, this was by far the most disappointing. We would not recommend this hotel to anyone wanting to book via the Hotels.com website.