You generally want to max E first, because upgrading it does not raise its mana cost, it reduces the cooldown, increases the damage, and increases the slow amount. When jungling this helps with clear and ganking, and, if playing AP Xin Zhao, should also be maxed for it is your main source of magic damage. The only time maxing Q first would be beneficial is if you are going to be pushing turrets, in which case each point in Q greatly increases your pushing potential.
Hope that helped!
-Cmdr. Sheperd