well it is a simple question. first we must discuss both weapons.
attack speed: very fast attack speed: very fast
damage: lower than katar damage: higher than dagger
level req: lower than katar level req: higher than dagger
conclusion: str dhans must use katars because of the higher damage and the 50 level buff "katar mastery".
agi dhans must use daggers because they don't need melee damage, so they aren't forced to spend much money on getting and refining katars, when they can get a dagger more easily and more chaeply
finally there is not a big difference