BLM

[Sandkat]

Thunder III cost DOUBLE the mana nearly DOUBLE the cast time, and only adds THIRTY potency. The Thunder Storm proc does add 100 potency, but it's a 5% chance, it's simply not worth sacrificing 1 Fire cast per damage phase.

It is double the mana but only 1 second extra cast time.

Thunder 1:
Cast time - 2.41
Mana - 212
Potency - 30
Dot Potency - 35
Duration - 18
Thundercloud Potency - 240

Thunder 3:
Cast time - 3.37
Mana - 425
Potency - 60
Dot Potency - 35
Duration - 24
Thunder Cloud Potency - 340

its also a 5% chance no matter what Thunder you use.

Also, if you only refresh Thunder during Umbral, you arent wasting any mana.

Blizzard 3 > Thunder 3 > Fire 3 > Fire 1 > Firestarter Proc > 500 mana > Blizzard 3 > Repeat.