It's situational (duh). You really shouldn't enfeeb exp mobs cause of the MP usage, and when you really need to enfeeb something, just test out a few things, you can cast more than once. If it's endgame mobs chances are you will need enfeebskill and staffs. if you are /BLm then you get a free super high potency slowII which is always nice :3 But, just use common sense...
XI Wiki

