If a "Tiras Keeper of Genesis" with 1 material left attacks a "Wind-Up Zenmaines", in the End Phase, End Phase mechanics apply.
From the above list it is clear that the Non-Turn Player is the one who chooses whether or not Tiras will stay on the field or be destroyed by Zenmaines' effect.
- Turn Player has a choice as to whether he will detach a material or not.
- Non-Turn Player has a choice as to whether he will select to destroy a card for Zenmaines' effect.
- Turn-Player has to detach a material from Tiras, if he hasn't done so before.
- Non-Turn Player has to select a card to destroy for Zenmaines' effect if he hasn't done so before.
From the above list it is clear that the Non-Turn Player is the one who chooses whether or not Tiras will stay on the field or be destroyed by Zenmaines' effect.