I never tried Charm.
At best, Assassin's Kiss has a 50% chance of doing double damage, for an average 50% bonus. In order for a knife to equal the best one-handed sword (71-100) it would need to do 47-67 damage. This is neglecting weapon speed; early in the game a knife would have the speed advantage (which would make up somewhat for the lower damage), but Sword Expertise drops the recuperation time to zero at later levels. I would think/hope the attack time would also be faster with a knife, but have not tested the damage per minute to check.
The general opinion seems to be that swords do much more damage than a knife/dagger and Assassin's Kiss. I started a survivor on hard, using a bow and knife (my first game was too easy, so I decided not to use any swords). However, I go distracted, then side tracked, so never made it very far in that game. The two potentially best knives in the game are in the chest in the back of Kistandalius' shop and the Nobleman's Dagger from the holy item quest.
