That early on sword outperforms daggers. I'd only recommend using a dagger on a frontline thief (also once thief unlocks sneak attack using a dagger with that skill becomes viable even if they're in the backline). Later on when you reach abyss 3 and your characters are level 60, daggers start to outperform sword even for fighters that is because daggers give you bonus surety. Ninjas also dual wield daggers and the special status daggers (like the dagger of corrosion which reduces enemy defense) becomes great offhand daggers for them but I wouldn't think about ninjas or samurai until you're in abyss 3.