Once you reach a certain price point, you don't get much better glass, but you can pay a whole lot more for a little, or in some cases, nothing.
The Z6 has no better glass than the Z3, you are paying BIG bucks for the zoom ratio. In fact, if the higher powered Z6 has more lenses in it, then it should be less bright. Thats "if", I don't know how many lenses each have. Either way, neither scope looks one bit brighter to me than a Leupold VX3.
Everybody is always hung up on "glass". There are a lot of other factors that are more important to me than simply "brightness" when selecting a scope.
Any high dollar scope is going to have more than ample glass for your task. I'd suggest doing a bit of homework and considering some of the other factors/specs..........