I would say that torpedoes must be fired more or less in the direction of the target, but missiles can turn soon after leaving the launch tube. Also, you can only load one torpedo per tube at a time, but missiles can be loaded on a rotating rack for faster firing rate. I also subscribe to the ST:TOS/ST:TAS and Star Fleet Battles view that photon torpedoes are energy weapons, not human-coffin sized physical objects a la Wrath of Kahn.