Can't be that much better than my ZAO!
I wish I could locate the article that I found the other day on this subject...
The author judged the contrast, resolution, and snap to focus with the Zeiss Mono to be clearly superior to that of a ZAO. He did not believe it himself at first, but after repeated assessments, he arrived at the same conclusion.
The Mono costs up to twice as much as a ZAO. Is the incremental but undeniable improvement in performance worth the additional outlay?
If only the best will do, and if you have the bucks, and if you have great vision, and if you have a killer triplet, and if you have great skies, etc, etc, then yes, this may be THE EP for you...
Was the Zeiss mono compared to the ZAO I or II? I owned the ZAO I set and sold it when I purchased the ZAO IIs.
Perhaps I'm simply not blessed with great skies, but to date, my 6mm Zeiss mono, 6mm ZAO II, 5.1mm Pentax XO, 6mm Supermono, and 5.9mm ball have all deliverered the same view of Jupiter. I suspect that an observer would have to have exceptional skies to determine a definitive winner in the contrast & resolution department. The prices these Zeiss monos are fetching these days seems to be excessively high given the minimal increase in contrast and resolution, if any, these EPs offer under normal skies.
If I had to do all over again, I wouldn't have spent bucks on the Zeiss 6mm & 10mm monos a few years ago. I'm still waiting for them to show their value with my skies.
To be fair about the 10mm Zeiss mono, I haven't compared it that often against other brands, since I've been using 4mm-7mm EPs for the past year when viewing planets.