I have both and shoot both regularly. I have had both on my 300 win and now it wears a 56. The difference in mounting height is less than .125" and it does not create any cheek weld issues for me on the Mcmillan stock and a fixed high cheek piece. All this is with a 30 moa rail too. I have had them both on different rifles at the same time and never noticed a difference shooting them, but the 56 does supply a little more light. Is it needed to shoot to last legal light, no, not at all. In fact if it is getting that dark I will not be shooting that far anyway. The only negative I have come across is getting the scope into a scabbard. It is tight. If there are elk hunts
with horses in your future, go 50. If you want that little bit of light, go 56. I can't say you will even notice a difference unless you have them both side by side for several comparisons.
Here is what NF says about the 56 over the 50.
"The ultimate long range riflescope. Its 56mm objective lens provides maximum clarity and resolution across the entire magnification range."
Hard to go wrong here with either.