Whips can be used from the back row with the same attack power. Being a spellcaster it's advisable for Rydia to be on the back row at all times. Therefore the Mist Whip is probably the better of the two. It also augments her Intelligence stat, which means more power to her spells, whereas the Assassin dagger adds to her Agility, although increasing her Defence Multiplier, Attack Multiplier and Magic Defense Multiplier (sometimes), being put on the front row will negate this somewhat. If you want more physical damage (and with about five really good front-line attackers I can't imagine why you would), then go with the Dagger. If spellcasting, backrow with the whip.