Man, these two knives are so entirely dissimilar I am surprised that the choice has come down to these two...they are so different in every way.
I admire both designs. Both are well crafted, but I use the D'Allara way more as an actual working knife, and prefer it's ergonomics and handling qualities in this role. The handle design is more grip neutral than the ZT in my medium large hands. The ZT is very comfy in a straight overhand (sabre) grip. Both knives feel very secure in the hands. The ZT is noticeably heftier by 2.5oz or slightly more, and physically larger and thicker. It definitely handles "bigger". I don't really mind carrying a big knife, but the ZT was a bit too much even for me. If you have to squat down with a ZT in your pocket you may find it putting a bit too much pressure on the 'ol boys. That said, the D'Allara is by no means small. It's just not as big.
For my purposes, the D'Allara blade shape/profile is more versatile, and it's hollow grind takes a *very* fine edge, so for straight cutting ability I find it superior. The ZT has more weight, and will chop more effectively, but that does not make it a machete. I am not a big fan of it's recurve blade for push/draw cutting wood. The "bump" in the blade disrupts the length of the cutting edge, so even though the ZT has a longer overall blade length I find the effective cutting length is actually less than the D'Allara. For this reason I'm not too keen on the ZT for general practical bush craft tasks. Also, that recurve blade will be a bit more of a trick to sharpen, so you will need some round stones on hand.
Locks...I like the D'Allara ball lock, real solid, strong and wear resistant. Not that the ZT liner isn't...never had it fail, nice and thick. It's just that a liner lock likely has a greater possibility of letting you down...especially if you treat it like a fixed blade or some such thing. That's more of a user responsibility issue, though.
But yeah, different design profiles for sure. The ZT seems more of a butch "defensive" knife. The D'Allara is definitely a working knife design, an efficient, versatile cutter. I'd go for the D'Allara if overall utility is the primary call of duty. The ZT is a first class knife, no mistake. If you want something that has that rugged tactical mojo, the ZT will probably give you goose bumps.
I would take a look at Spyderco's large Manix or maybe the Chinook...big tough knives. Both cut better than my ZT, and they are closer in terms of heft and beef...and price.