"Easy" or "hard" are mostly matters of opinion, and in leveling it doesn't really matter. All three specs work just as well for it, because it's, well, leveling, i.e. by definition not the challenging content of the game. In raids, all three specs can be challenging depending on the fight, because all have a variety of mechanics with which to min-max. Wrath destro was also not really much easier than demonology.
DoT specs are overrated in difficulty. Yes for utter newbies they are harder, but once you know how to play the game they're not. For leveling I went with demo, but destro also has powerful AOE potential and also served me well. Explore all three and see which suits your style.