Assuming equal quality scopes 50mm objectives offers better usable light transmission over a 40mm scope only between 8X and 10X magnification. Anything over 10X and there is no difference. Anything below 8X and the human eye can't use the extra light.
Assuming equal quality a scope with a 50mm objective will cost $100-$200 more than the same quality 40mm scope. At the same price point a scope with a 50mm objective is a lower quality scope. I've found that moving up and paying $100-$200 more for a better quality scope with a 40mm objective gives better results than spending the same money on a scope with a 50mm objective
