Although I don't own any Zeiss scopes, I have used many of them (mostly the expensive VM/V series). As for the Conquest, I believe that the glass is better than the Vari-X III. Then again, my Elite 4200s and Nikon Monarchs have better glass than the Vari-X IIIs. The Vari-X IIIs used to be the benchmark about 12 years ago. Since then, however, the technology has remained the same and Leupold hasn't done anything to better the Vari-X III. Sure, they came out with the LPS - which is a super scope by the way, but really nothing optically for the Vari-X III. The Monarchs and Elite 4200s, on the other hand, have gotten much better!
Okay, back to the Conquest. It's glass is about the same as the Elite 4200. Another benefit of the Conquest - which I REALLY like - is that it's reticles are etched in the glass - now that's the sign of quality. Swarovski and Nightforce NXS scopes also have this feature.
As for 30mm tubes, the benefits (the real ones anyway) are 1) they are stronger, and 2) allow for more internal adjustments. Many people think that 30mm tubes increase light transmission. Well, that's true to some extent, but not as much as you would think.
Let me tell you somthing - if you really want a tough-as-nails scope with super optics, the get the Nightforce NXS. I have one in 5.5x-22x-56mm. It has etched reticles and is illuminated. It's HEAVY, but it ROCKS!! It also costs about $1,200, which is a little bit more than the Leupold, but THREE TIMES THE SCOPE!! And Nightforce scopes arguably have the BEST ILLUMINATED RETICLES on the market.
One of the reasons that the scope is so heavy is because the tube is THREE TIMES THICKER than your average scope. They use this scope on the .50 BMG 1000 yard competitions. I have this scope on my Remington Sendero SF in .300 Win. Mag. and it's a perfect match (I don't, nor could, carry this rifle around the mountains all day.)
As for tactical applications, it has all the bells and whistles. With the excepcion of weight, nothing beats a Nightforce NXS - not even Swarovski. Granted, some people think that the Swarovski (PH that is) has "truer" color, but I haven't noticed it.
I bought my Nightforce from Chris at
www.SWFA.com about a year ago. Call him and ask him what his thoughts are on the Nightforce. I don't know if the optic zone has Nightforce scopes, but check and see. See if a dealer in your area has one - you will be impressed.
Zachary