Z1 - Much much more economical.
ZFX - Ability to layer up to 3 coats in a day.
As for durability, I haven't seen a difference between the two but then again, I haven't gone 6 months without applying another layer of protection. When I do (mother's vehicle), I minimally apply 3 layers with ZFX so I personally don't know if Z1 / ZFX effect's durability. I would suspect it doesn't.
Also, using Z1 minimizes the amount of Z2 of Z5 you waste. If I plan for 3 coats using ZFX and end up only doing 2, I end up having to discard 1/4 of ounce of product that has been mixed. For me, it's not a big deal, but for some I guess it could be.