i am also thinking about these two models.
i don't really like 5s, but given that my current contract will end soon, and i want to get a new phone anyway, maybe 5s is easier for me to migrate as i am using 4s and i have many apps and data (e.g. password app) that i still want to keep.
i also like iPhone's camera more… the photos look more natural
but i think z1's screen is quite nice. much better than korean phone