Vers is always just increased damage... just happens to be more damage with a minor damage reduction attached.
If mastery is just extra damage, and vers is near the same damage increase with a bonus... why not use it? If the modifiers are not similar then it's not a viable option, but if they're really close then it'd be silly not to utilize the extra reduction.
I never looked, but say MM gets range and damage with mastery vs. damage/reduction of verse, that's a no brainer to me for the extra range.... no idea if they're similar tho, just saying the extra utility is the decision-maker, for me at least.
I thought he was whipping around double CoCs the whole time, I didnt realize that the first was the nova...