If you just want to beat the game, kill Ornstein first. If you want to [i]perfect[/i] the game, kill Snorlax first. This is because Snorlax is easier to kill, so you want to kill him first so on your first playthrough you can fight the hard guy and not have to fight him New Game + where he's much harder. This way you get both armor sets easier (whomever you kill last, their armor set becomes available for purchase near Firelink).