The VX-6 3-18 weighs in at 574 grams, the Z6 3-18 weighs in at 595 grams. For the $1K more for the Z6, I'd go with the VX-6 myself - lighter weight and I doubt there's a significant difference in glass/coatings/clarity/low-light etc. At least not $1000-1200 difference.
Both should do what you want. One just does it for a significant penny less.
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