Imagine the below situations :
1) Tank is left with 1k hp, you cast Cure 2!! You see your cast bar finishes, casting animation complete!! But....tank still died. If only I cast abit sooner/my casting speed is slightly faster..... (SS wins in this situation)

2) Tank is left with 1k hp, you cast Cure 2!! Cure 2 heals tank for 1500 hp!! But Boss crits tank for 2515 hp, tank died. If only I have more DET so I can heal slightly more!! (DET wins in this situation)

3) I am the bestest healer of all time and eternity, I can anticipate every hit from bosses and my tanks never die on me, their hp is always above 50% (It does not matter if you're going SS or DET, since you already bestest anyway)

Personally for me, I find Situation 1 happen to me the most (excluding Situation 3 of course :P), and that is why I choose SS over DET .