I have been using both for a few years now. I see no real difference other than the 56 takes a bit taller rings. As for looking through them , I really cant tell a difference. You really can't make a bad decission here. Just get which even you feel will serve YOU best. Either will be awesome.
Mine is on a carry rfile and I got the 50. Advantages are a little less bulky and lower mount. The 50 gives me plenty of hunting/shooting light, morning or evening. I agree that it will be hard to go wrong with either.
+1 on the50mm, dont see the need to go to 56mm
As you know, I have shot out at " the range", and have shot in low light out to 720yards with the 50mm when I could not see the target with the naked eye.