I've never used a Vari-X III so I don't know if there is a difference or not.I will say I have a few Vari-XII's 3x9x40mm and have been quite satisfied with them.I have one of the Original Redfield Illuminator 3x9x50mm. To me there is NOT a nickels worth of difference between the bigger objective.Unless that money is burning a hole in your pocket I would go with the 40mm.Another thing with the 50mm you'll have to go with higher rings.I like to keep my scope as low as possible.
Ruger Redhawk