What are the departure times on the two IAD-xxx options? I have a heavy preference for a later departure, especially when I'm in J. I loathe flights that leave at 5pm and arrive early in the morning are horrible, because I can't get much sleep. I'd much rather depart IAD/EWR at 9pm, go straight to sleep, and land in Europe at noon.
(Actually, the best TATLs for sleeping, IMO, are the ones that leave SFO at 7pm and land at 3pm Europe time, but that's clearly out of scope here).