787-8 and A380 CW seats are practically identical. The A380 lower deck is wider, so has a higher proportion of middle seats, so in theory you'll be less likely to get a window, but not by much*. If you're fine with either airport then I'd just call up and see if the IAD option is cheaper, if it is then take that, otherwise just stick with the 787

* Edit: Actually, looking at the seat maps that's not true, the A380 is wider, but there is an extra set of window pairs without a corresponding set of 4 middle seats, so the proportion of windows to non-windows on the A380 is about the same as the 787