Don't want to throw too much at this question, because united trips never seem to be predictable...have done the DEN/LHR run about 10-12 times over the last 2+ years, used both IAD/ORD equally IIRC. Sleep is an issue from IAD eastbound, but as cited weather is a problem at ORD all the time. Customs is a real crap shoot, both seem to be fast, but the luggage delivery is the real independent variable. I have had my best and worst speed at ORD, w/no predictions possible. I have also had to hoof most of the way thru C/D at IAD -- easily equal to the train connection. All said and done, in Nov. i would avoid ORD no matter what.