After owning several 56's and then moving on to the 50's, I can honestly say I personally have no desire to ever go back to the 56's. Even 50 is a bit on the big side but IMHO, it is much less bulky and a tad lighter. It also affords the shooter is some situations to use lower rings which further reduce weight and keep a lower profile on the rifle.
As far as lighting, Either will offer MORE than enough light to make an ethical shot even before and after legal shooting hours in normal conditions. You may have to reduce the power to get the best light transmition. Both my 50mm NF and my 40mm Mark 4 have enough light capabilities to shoot a sheep at 12:01 AM on opening day which is a tad darker than 30 minutes after sun set in any of the lower 48 states.
BTW, I am not encouraging shooting game past or before legal shooting times. The point is that either will work at legal light. Up here we do not have restrictions on legal light. As long as we dont use night vision, if we can see it by natural light we can shoot it, regardless of when the sun set.