With the exception of BA and CX, I always travel on the lower deck. Lower deck is often less crowded (my flight the other day was 26/26 upper deck and only 10/21 lower deck). Plus larger overhead bins on the lower deck so easier for storage.

However with CX (prior to the latest new business class) the upper deck seats were 6 or 7 inches longer when converted to beds than the lower deck, so upper it was. And with BA the 2+4+2 is a bit crowded for me on the lower deck.