KARFA

The only issue for the UD windows is that the fuselage along the UD is curving and therefore the view down can be a bit more restricted as the window is slightly angled towards you. Main deck windows are not similarly angled so if it is purely for the view I would suggest main deck, and a window seat in the mini CW cabin behind first so the wings and engines don't get in to the view.

Having said all that, for me all the others advantages of UD outweigh the slight disadvantage of the view when compared to the main deck.