How can the seats on the 787-8 or 787-9 be any narrower than the 767 one? In flat bed mode I could barely breathe. And I am of normal weight and height!
I don't think they are narrower. But I recall that the 767 seats have one armrest that can be lowered, effectively making the "bed" position a bit wider in that spot.
I'd pick the 787 any day all things being equal, but I wouldn't add hours to my trip or many dollars to my fare, or have a connection I don't want to get on the 787 over the 767.