Infusing with faint stone vs. Infusing with Magic stone?
The faint stone adds magic damage scaling with your INT, and the magic stone adds physical damage scaling with your INT. Both reduce other scalings and base damage. Also the faint stone can be used on shields but not the magic stone.
All else being equal, the magic stone adds less damage than the faint stone but since it's purely physical you don't have to deal with multiple resistances.