If you are primarily concerned with sleeping, the window seats are probably the best, being the most private. Just keep in mind that you have to climb a bit to access the aisle, and there is a missing window at row 9. The middle pair, seats C and H, are probably the most desirable, as they have direct access to the aisle, and no one will be climbing over you. The LHR to ORD flight should be mostly in daylight, so not as many people want to sleep during the flight. You might want to try one of the window seats on your IAD-FRA flight, and a C or H seat on the return.