Let me to explaining this after a nights sleep:
Enemy is standing in front of you and tries to run away. With StandStill you can stop their movement and with ISB you can move them 5'. I suppose at that point its the person making the AoO deciding if they want to use the ISB on you.
If you have 10' of reach and they charge you. With SS you get an AoO but can prevent them from entering your threatend area. If you use ISB do they end up 15' or 20' away?