Re: 50mm or 56mm is it bright enough to make it worth it???
We all have various interpretations on features. Some are more important than others.
This guy wants to know about 56mm objectives, and how that contributes to so called "brightness", not lens coatings or internal dimensions, or even assurances of what is good enough, under "legal light" conditions.
The straight answer is that 56mm is an advantage for shooting in extremely low light conditions....all things being equal. Meaning: same internal construction, same multi coatings.
The only caviat, aside from the premium price, is the height of the mounting to clear the barrel. If you are okay with that, you will see more detail during low light, with the larger objective. period.
Good luck with your choice, LB