I guess you want a comparison as mage DPS. But do you need an off-healer or an off-Time Mage in your Roster ?
Did you already invest in one or the other ?
Will you use auto or manual mode ?
Y'Shtola will have an easier time to MaxLB (there is a FF14 rerun in JP, so 4 months away from now in GL).
As DPS, Y'Shtola is better.
Set her sub command to black mage. She loses the haste sadly, however she will go all out full damage after that. She is a very solid fire hero option if you do not have Mediena, and are not building another fire hero.