Are the extra days over a weekend (Thurs-Sun night) or during the middle of the week? If you're paying for a weekend stay, I would go with the cheapest one. If you're using points for a weekday stay, I would recommend the HP Dulles East. That property is the closest to the Air & Space Museum, and they run a hotel shuttle there. The hotel has washing & drying machines for $1 a load. I make this recommendation assuming that you have access to a car so you can leave the hotel to eat. Otherwise, you'll be dependent on the hotel shuttle to drop you off within 3 miles for food. There is a large strip mall about a mile away from the hotel with a lot of restaurants and a grocery store.
The HP Dulles South is located in a small strip center with a few restaurants. I have had a few stays here, and they were fine. I have not stayed at the HP Dulles North.