I just ordered the Fast Fire II (should be here Friday). I have never used one. However, I have used scopes, and have two EoTechs on my dedicated turkey guns. I prefer the Holographic sights, as it is easier for me to keep both eyes open, and see my peripheral vision better.
I'm hoping the FF II will do the same for my daughter's gun. I also chose the FF II due to size and weight for my daughter's gun.
I would PREFER the FF III, but our youth season starts March 10th, and I want her to be comfortable shooting the gun.
Just my 2cents, I'd go with the FF.
God Bless,
David B.