Have you ever compared them side by side on an optics chart? Everyone who has looked through my scopes at the chart agreed the Swarovskis came in second. Since I bought them, as well as as others, I wanted the z5's to be the best since they cost the most by far. Not only that they are four ounces lighter. You might be surprised that even the Leupold VX-6 4-24X52 beats the z5. But it's gone because they are even two ounces heavier than the 6500.
I really do like the 1" tube of the z5 and really disliked the 34mm tube of the VX-6. I mean the 30mm tube of the 6500 is not necessary for the vast majority of shooters.