Spell Damage% vs Rainbow elemental damage %

So ya I wanna know which one better since I can make crafted consumable
That can have 30% all elemental damage but also can make 30% spell when I say better I mean which one would provide a better damage buff

I believe its a build depending so ya can anyone tell me which one better where both would be effective on ETW or water / earth focus weapon like idol / alkatrz

 

if your spell damage% is lower than water/earth damage%, use the spell damage consumable, otherwise use the elemental damage one

 

ty