I prefer the 40mm objective lense scopes as they are lighter and can be mounted lower than a 50mm scope.
Light transmission has to do with the quality of the lenses and their coatings.
Exit pupil determines how much light gets to your eye, and is determined by dividingt he objective lense size by the power setting.
So a 40mm scope set at 3 power is 40/3=13.3mm and 40/9=4.4mm, whereas the 50mm scope is 50/3=16.7mm and 50/9=5.6mm. So the larger objective lense will allow more light to enter your eye under low light conditions and will appear brighter.
The down side is, it is larger, heavier, cannot be mounted as low so may affect your cheek weld on the buttstock when trying to center your eye behind the scope, and can be bumped out of alignment easier as it is not as slim compared to your rifle's profile.
This being said, the human eye pupil can only dilate to a max of 7mm when we are younger and the pupil muscles are more elastic. As we age, this elasticity deteriorates to the point that by the time we are in our 60's may only dilate to 4 or 5mm. So regardless of the scope's exit pupil, your eye may not be able to accept all of the light coming through the scope and will affect how bright the scope seems to you under low light conditions.
So for a younger person the optimum power setting for the scopes under low light conditions would be just under 6 power for the 40mm scope and just over 7 power for the 50mm scope.
A common myth out there is that a 30mm tube scope allows more light through the tube and ii brighter. This in fact is NOT true, as verified by the engineers from a variety of scope manufacturers!
As with all things, you get what you pay for, and the quality of the glass you buy is no exception! But as always, budget comes into play for many of us. Buy the best you can afford!
Hope this helps you make your decision!