Here is something to think about.
Incoming light is a direct relationship with the size of the objective lens.
The larger the lens diameter the more surface area there is to receive light. Optical coatings will help and the type and quality of the glass will help. But in the end, surface area is a big part of the ability of the scope to deliver a clear sharp bright image.
So, we remember that the area of a circle is equal to PI* r squared
Using the radius of the scope objective lens we can make some calculations.
A 40 mm scope has a radius of 20, a 50 mm scope has a radius of 25 and a 56 has a raduis of 28. Squaring each radius we get 400, 625 and 784.
A 40mm scope has a surface area of 1,257
A 50 mm scope has a surface area of 1,964
A 56mm scope has a surface area of 2,463
So by going from a 40 mm scope to a 50 mm you get a 56 % improvement.
Going from a 40 mm scope to a 56 mm you get a 96 % improvement,
You will notice that the last 6 mm of size in the Nightforce delivers a big improvement over the 50mm scope.
Lastly, and I will leave you to your headache after this, The NPR1 and R2, which I have both, are very fine lines and very difficult to see the last few minutes of the shooting day. I have had the opportunity to shoot a buck with them with still about 5 minutes of shooting time left and it is just very hard to see the reticle. I have not ever tried the illuminated reticle. The very fine reticle is really great for getting a good aiming point but it has its problem right at quitting time against a dark background on a overcast day down in a valley.
My advice is if you can afford the nightforce it will give you a big edge on those bad days when the deer don't come out until the last minute. Secondly, I would go down under the optics part of the forum and make a thread "Night Force Illuminated reticles" and ask for real world experience using them either in the dark or as darkness closes. You want to know if the illumination causes a loss of target clarity (i.e. can you still see the deer or just a bright light in the scope). Or perhaps you can run the Search function and find the answer.
I have tried to provide you advice based upon my personal knowledge and we have just run out of my knowledge.