First off if you want to stay with the low mounts you'll have to get a 40mm. The difference between the VXIII is index matched len coating (Light transmission higher, brighter and clear than the VXII), finger adjustable windage/elavation & fast eye focus on all VXIII. Some other features can be had as well that the VXII does not offer (optional). Personally I would opt to spend the extra on the VXIII, but the man with the check book must decide in the end.

Good Luck