What's the difference between Miss Fortune and Mysterious Stranger?

Miss Fortune appears randomly when attacking in V.A.T.S, and has a roughly one in three chance to use each of her attacks, they are:

  1. A Knockdown against your current target
  2. Disarm your target - the weapon will be flung across the room. If your opponent is unarmed, this result will never occur.
  3. Cripple a random limb. There is a higher than average chance of this being the torso, and a slightly lower than average chance of this being the head.

By contrast, the Mysterious Stranger will only appear if, at the end of a V.A.T.S. attack, a foe has less than 150 HP to finish them off with a headshot from his .44 Magnum.

The chance of an appearance for either is 10%, though this can be doubled with the Friendly Help perk, acquired when you receive 15 visits from either the Stranger or Miss Fortune.