My working theory is that the dominant factor in view of the reliability of the two hubs is whether one reaches them mainline or UX. In PVD, I can go ORD mainline or IAD on UX. I experience far more cancellations and delays at IAD (although I also think it's a far less pleasant terminal) because when whether gets a little dicey, they cancel/delay the UX flights before the mainline flights.

My advice is to use whichever is served by mainline, if you have that option.