I have an NEF shooting system with both a 20 & 12 gauge slug barrel. I've shot the 12 gauge TWICE and put the barrel up. Recoil was just too much for me and I'm a big guy-6'3", 240 lbs.
And from EVERYTHING I've read, seen and experienced the 20 gauge is more than enough gun to put down a big whitetail.
My little 20 gauge NEF rifled bull barrel is really accurate out to 125 yards! No brag, just fact.