The two mages are distinguished by their abilities and their channeling. Saying that the Priest is more aggressive while the Priestess is more defensive is a fair assessment.
Priestess
Greater channeling gives her an ever increasing advantage with the longer that the game lasts. Her ability to increase her Life also favors a longer game. Finally, her Restore ability allows her to provide excellent support to her creatures.
Priest
Lower channeling means he favors a quicker game than his counterpart. Holy Avenger is actually a defensive ability designed to give your opponent a Catch-22. They can either attack a very tough creature (+5 Life) or they can suffer a punishing retributive strike (+2 Melee). Finally, Malakai's Fire gives the Priest the most devastating melee attack out of any mage. His basic attack does around 4.5 damage on average (against an unarmored opponent) with a 1 in 3 chance to Daze. Meanwhile, if you equip him with the Staff of Asyra, he will do more damage against an armored target (2+ armor) than a Forcemaster with Galvitar will AND have a chance to Daze or Stun his target.
Comparison
Since he has the most powerful melee attack out of any mage in the game currently, I think it is clear that if you are wanting your Mage to be your primary damage source then you should pick the Priest. If you plan for most of your damage to come from creatures, then the Priestess's Restore ability is arguably better than Holy Avenger, but I don't think there's enough of a difference to say that picking a Priest with such a strategy in mind is a bad decision. Especially now that we have Wand of Healing.
The main difference between them, however, is the length of games they favor given the Channeling differences and Divine Reward. With the Priest you want to be aggressive. He can hold his own with a mid-range strategy, but he typically does not want the game to drag on too long. Meanwhile, the Priestess can get stronger with the longer the game progresses if you play her that way.
Thus, if you want a shorter more aggressive game, or you want your mage to be dishing out the hurt personally, pick the Priest. Otherwise, the Priestess is probably the better choice.
