The FF II battery compartment is underneath the sight requiring it to be unscrewed from the mount to replace the battery. The FF III battery compartment is on the top so that battery changes do not require you to remove the sight from the gun.
I think that the FF II is also auto brightness adjustment only and the FF III has auto adjustment and 3 brightness settings. The FF II weighs 1.6oz and the FF III weighs 0.9oz. The FF III also has a slightly smaller dot at 3moa than the FF II with 4moa.