It amounts to around 4-6% more light transmission capabilities with the vari X III (95%). The VX II I have heard is 88-91%. Multicoating on all lenses will lead to better optical quality all around, this includes light transmission, clarity, resolution & brightness. The light gathering capability should be the same if the objectives of both models is the same (FYI, 50 mm objective equals about 2% more light gathering characteristics than a 40 mm objective in a comparable scope).