The ball turret gunner had to curl up into almost a fetal position (sperry ball turret type; not the periscope gunner tub type), while every other gunner was either standing or sitting/kneeling at their station. I wonder if this helped the ball gunner present a smaller overall target than the other gunner positions, though any advantage this gained was probably offset by the position's exposure to enemy fire due to its location on the airframe.
Personal preference would be waist gunner position for perceived survivability and ease of bail out ability, and nose gunner for perceived kill earning; but that is all personal opinion based on some books and conversations with surviving vets of the era.