Destruction is more centered around dps, and is often used for leveling. Affliction is a little more centered around curses, so many locks wait til 60 and/or raiding to start focusing on that tree. Demonology is fun, too, though. But it depends on your playstyle and what you spend your time doing the most. If you're looking for a straight leveling build, then I'd personally say to go Destruction. If you like to pvp, etc, then Affliction. If you like what Demonology has to offer, then do that.
