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.