As far as I know, as an aion player, the priests are the best classes in terms of leveling speed before level 10, solely because of their abilities to heal themselves in combat. In solo combat, their battle style is mixture of both Smites and heals. This enables them to keep their health up while killing enemies. Moreover, they will have better understanding of the features of both the Cleric and Chanter. This allows them to better prepare them for the choice of which class to take after their Ascension. When in group combat, it is their goal to heal allies while once in a while making an attack or two.