Demonology helps with the use of a Voidwalker at early levels, I always leveled Affliction, but I think Destro is a bit more gear-oriented. Pick one you like, learn it, love it. You can always swap around while leveling to find what you like best. Any of the 3 should work fine while leveling.
Im currently leveling a warlock and have been destro since the beginning. I tried some affli around 55 or so but quickly changed back to destro. Its just my weapon of choice! Works very well in LFR, questing and PVP too. Im currently 65 and mobs go down like flies.