She's a jack of all magics, master of none. Sub red mage over black mage at 120 for versatility and no redundant fire magic. She does less DPS than salire and ellshra, higher damage than most non BM MRs like Chel, Helena but outclassed by ellshra as TM w' DPS. She is generally good since she can quicken, cure with red mage, has innate speed cast and gives it to stronger AT MAG DPS, chains fire, ignore fire res w' meteor, and inflict berserk if the enemy resists MAG and fire.
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.