The mana on-use is not going to be useful for you in a 90 second fight. Robes of Volatile power give slightly more damage. They are really so close in stats that you’ll never notice a difference.
The calculations we do look at your mana pool, so it does a balancing act as far as where the most efficient place is on your gear to get more mana through intellect, spirit, mp5, on-use, etc.