A knife is a knife is a knife. IMO, if you want a flipper and/or a frame lock, go with the ZT. If you want an Axis lock, go with the 710. Personally, for the price, I might suggest looking for a 710 in M390 if you are considering the ZT as an alternative.
My personal experiences: my M390 710 had to be sent back for warranty work because the blade tip was awful close to sticking out of the handle when closed. My ZT (different model) in S35VN was good for 3 cuts on R30 fiberglass insulation and then became dull enough to run down my hand without cutting. I then proceeded to cut the remaining 30 cuts with one blade on a $1.99 box cutter. In all fairness, I'm sure most of my other knives would not have held up much better.
I love them both and wouldn't choose between them... therefore I might suggest getting the discontinued model first.