HR Reston gets cheap around Christmas, and it is a good time to stay there since you'll get the 2500 points for the closed lounge if you are a diamond member.
I disagree with Fair Lakes and Dulles being the same. Fair Lakes has panoramic suites that are quite large at the ends of each floor, but the breakfast buffet and hotel gym are inferior to Dulles. Hyatt Dulles also has a nice business center with free computers for internet use, printing & copying. Hyatt Fair Lakes charges if you need to print anything or use a computer for internet access for more than 15 minutes.
The basic rooms are nicer at Hyatt Dulles, and the toilet paper is softer as well (Fair Lakes does not use Cottonelle).