Hi ormondprop,
I have recently bought the 56mm one and it is the DOG's testicles, it makes the NightForce I have look like a cheap scope (well not quite, but the Zeiss is much better) the 56 and 72 will be the same in day light the only thing the 72 will do better would be dawn/dusk.
However, my 56mm is very good at dawn dusk and much better than the N/F. At night with a lightforce SL170 lamp with Amber filter I can pick out a single leaf on a tree at over 300 yards.
My advice would be go for the 56mm the mounts are cheaper and you can mount it lower to the barrel, but the choice is yours.
All I can say is if you buy one you won't regret it.