For very low light, say at or past 30 minutes after sunset, an exit pupil (EP) of 5mm works pretty well for me, generally speaking. The assumption here is that your own eye's pupil will open up that far. So a good 50mm riflescope scope at 10x or 10x56 binocular fit the bill for my low light use. Or for that matter a 42mm riflescope at 8x will get the job done--it will show more detail at 8x than 6x--somewhat dimmer, granted...). An excellent 42mm will do all you need it to in most low light hunting, but a larger objective in the same quality glass will do it a little better and you will find advantages with a 50mm or a 56mm--like Crow & Bobby said.
Power is an asset to the "mature" hunter. Greater power requires more objective size to keep a given EP, and at some point there is a limit to what you want on your rifle or willing to carry. The less light you have also puts a practical cap on the range. For me, some field of view around the critter is helpful, and 10x at 200 yards works pretty well in that regard.
If you look through enough scopes 30 minutes past sunset or under moonlight, you see there are differences--some are subtle and some aren't. Some are better earlier in twilight and some are better later. Sometimes the eyebox gets fussy when the light wanes. You have to compare them side by side or take copious notes under controlled lighting. A lab would help... For me, I compare side by side for the best performance in view--what the glass will show me in small differences over its brethren--the one picked may not necessarily be the brightest. And the rub is, a lot of it may be subjective to the viewer.
Overall the differences are small and when guys reports that a certain glass adds 15 minutes of time to their hunt, they were either selling a product or did the comparison at different times under different light conditions.
Swaro, S&B and Zeiss have been making scopes for shooting porkers in the dark for a long time...and to make sure all your cash is spent, personally I put far more emphasis on the quality and low light view of the binocular over the riflescope for results.
fwiw...
