I'm actually trying to make up my mind on the very same issue. The only difference I see between the 50 and 56; the 56 comes in 1 ounce heavier and the eye relief of the 50 is 3.7 in vs. 3.9 for the 56. Basically, there is no difference.
The field of view is actually the same between the 50 and 56. I own 3 NF Scopes all 56mm, all of which are the bench rest models(end adjustable objective). I have all of these mounted w/ extra high rings. I use Eagle stock packs on all of my rifles and get a great cheek weld with these packs.
However, this time I plan to go with the NXS 5.5-22X50. I hope someone will chime in with their opinion on the light gathering issue between the 50 and 56.
"GOD, GUTS, AND GUNS MADE AMERICA FREE" AND I'M KEEPING ALL THREE!