For leveling purposes.
Mage is generally Offensive.
Cleric generally Supportive.
If it's AoEs, then Mage is the choice plus the class is suited for attacking.
Ideally it's best to stay on one skill tree to get the optimum "benefits" from the class.
If you prefer battling, then you should make a mage. Thus aoe implies mage. However, cleric is the most useful class at first because you can use it to level other characters faster (by opening 2 accounts at the same time). If you choose cleric, only learn supportive skills.