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.
If you want to level mainly through lfr, go destro until lvl 67, demo then.
If you want to level solo, go demo until lvl 80, then you can choose between demo and affliction.
Take the one you want to play on max level. Went fine for me (Affliction) - although I tried out the other specs, only finding me going back to Affliction.
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.