The good news is on recent scopes (I bought mine new last fall, manufactured sometime between July and September) Tangent Theta has definitely fixed the stiff mag/parallax rings. The bad news is that it took seemingly years of feedback for them to implement that change on all new scopes (though I heard they would adjust it for you if you shipped it back to them on the old scopes).
Between the two my decision wasn't at all based on the glass clarity, because the difference between the two is honestly so small that it may as well not exist at all. The only place there's really any noticeable difference in resolution/clarity is if you're using max magnification at 800+ yards when mirage starts to come into play. You won't notice it at normal PRS/hunting magnifications (18x and below) and you won't notice it for the majority of your shooting inside 1,000 yards even at max power. The primary difference between the two is just the color and contrast based on the lens coatings for each. ZCO has a bit more contrast and that makes colors feel a bit more "vibrant", so to speak, while the TT feels overall slightly brighter with a bit less contrast. Not washed out on the TT, not oversaturated on the ZCO, just slightly different balances of color, brightness, and contrast for each and I think that's really the only thing that makes mirage more visible on the ZCO compared to the TT at high magnification.
I bought the TT not because of glass though, I bought it just because of features. I like the Tremor3 reticle, and I really love the tool-less rezero functionality on the TT. I swap my scope between a lot of different guns, which means I adjust the zero a fair amount. I also like to adjust my zero each time I go out shooting, if it's a tenth off here or there. Being able to do that without having to dig out my tools is a big deal to me, but it doesn't mean much at all to other shooters who won't move the scope around as much as I do. I also personally don't care much for locking turrets, but other people love them.
You won't buy either scope because of their optical performance compared to one another. They're both head and shoulders above every other manufacturer, and they're near enough equals that only the snobbiest of glass snobs would truly prefer one over the other (and even then, it would depend on the balance of color, brightness, and contrast they prefer rather than one being objectively better than the other). Buy the one with the features you like - for me the T3 reticle and tool-less rezero were worth the additional cost. For many other people the MPCT reticles and locking turrets are considered upgrades over what the TT offers.
Get the features you want and you'll be happy no matter which scope you purchase. For me personally if ZCO ever offers a scope with a T3 (or similar) reticle and tool-less rezero I'll sell my TT immediately to buy one and spend the price difference on other gear.