At ORD, the traveler would need to change terminals via the train after clearing customs and then re-clear securty. The upside would be getting a mainline flight to DTW.

At IAD the traveler would likely still need to change terminals to get the United Express flight to DTW, but its less of a hassle than at ORD.

Either airport could be subject to summer thunderstorms, but I'd still probably pick ORD in this case.