To keep this a short-answer question... Besides the drop in display size and the mods-approach, is there anything about the Moto Z that strikes you as being an advance over last year's Moto X Pure?
I never quite did figure out why last year's phone just didn't do it for me. I think maybe just just chose a bum back cover. But I LOVE how easy it is to change that on the Z. And I LOVE the implementation of the mods. They're really well done.