As for leveling experience, single box and dual box are totally different. Holy Priest by himself: good luck leveling with that. But once you're paired with mage, you can level ultra fast through AOE grinding.

I've been leveling pally + priest and so far it's great. I can pull 5-6 mobs same level, and I simply can't die. Although the damage is a bit low, so leveling is a bit slow. Going to try mage + priest next.