I tested this on an iPad Air 2. Both options work, but for some reason the Sprite option makes Safari prompt "This page is trying to open a new window" with options to "Allow" or "Block". I think if you block it, it never asks again, so you might think it's failing. However I found a workaround: if you use "on any touch end" followed by "is touching Sprite", it can open the popup without prompting.
I think this is related to Safari change to do with what counts as a user input event. When iOS 9 came out we had trouble with audio no longer unmuting (Safari needs a user input event to allow audio playback). They blocked "touchstart" from unmuting, but it still worked in "touchend". So we moved unmuting to "touchend" and it fixed it.
So the three approaches are:
"On button clicked" - triggers in a "click" event - Safari allows popup
"On touched Sprite" - triggers in a "touchstart" event - Safari 9+ blocks this
"On any touch end" - triggers in a "touchend" event - Safari allows this (combined with a "Is touching object" you can get the same effect as "On touched sprite" but triggering at the end of the touch instead of the start)
So you should be able to work around this already with the "On any touch end" approach.