One question is where do you need to be "in the DC area" ? Both Dulles and Reston are some distance from DC itself, and depending on the time of day, the drive can be close to an hour or more.
Dulles and Reston are 5 miles apart and the time difference to IAD is almost immaterial, less than 5 minutes.
If I'm not spending time at the hotel (other than to sleep), Dulles is fine. I prefer the breakfast in the restaurant over the lounge at Reston. There's more to do - eat, shop - in the immediate vicinity of Reston, but that's its primary attraction for me.
On the Wednesday afternoon/evening I'm in Georgetown, and on the Thursday I've got to be right downtown. I don't plan to spend any time at the hotel other than to sleep. My reason for choosing the IAD hotel over Reston (where I also don't need to be but appreciate that there's a bar or two I could stick my head into for a nightcap) is the proximity to the airport for the Friday early flight. I know neither of them are close to central Washington and that neither public transit nor Uber/taxi is a great choice, though I didn't expect that the IAD hotel is actually not that easy a commute to the airport itself.
I guess what I'm trying to decide is, if the Reston property is significantly nicer and the airport and city commute is no worse, maybe I'll just stay there.