I would prefer longer flight in Polaris, but in this case I will choose ORD-LHR for the following reasons:

1. 767 is one of my favorite aircrafts and I try to fly it as often as I can before it goes into retirement.
2. Saving miles is always a criteria for me
3. Flying Polaris, I have been turned off by the SFO Polaris Lounge services, offerings and crowd, and would prefer to spend time at ORD Polaris Lounge if I can plan a longer layover.

Assuming you are flying Polaris, If you can squeeze IAD-LHR into your itinerary, IAD Polaris won't disappoint you. if you enjoy visiting lounges.