Noob Question: Which gives faster XP gains, dungeons or quests?

Hi reddit! old player returning to the game, I have a question for those of you that have been leveling lately. I've noticed that this new random dungeon thingy is really great for XP, My new rogue is already level 40! the question is, at what point do I leave the dungeons behind and go back to questing? Which is faster for leveling, questing or instances?