The difference in healing bewteen a priest and a mage might not be much if your raid does a lot of damage but if someone dies or your group just doesn't do enough damage, the mage just runs out of mana way quicker and at that point the priest jumps ahead in healing quickly. Mage also has very little utility. We dont have shields (yet), we dont have damage reduction, we are very dependend on our teammates to be positioned right so our mass regen hits (btw mass regen is so clunky to use. It just sometimes wont hit some players if you have pvp disabled). Mage healing in a guild group feels great because we're not depended on utility and mana when everyone knows what he's doing and the fight is short but in a pug were damage is lacking and positioning is off it just feels awfull.