As stated, slightly better work rules at NK. Yeah people may not normally drop to zero, but having that option as a line holder is almost like an insurance policy. The companies operate essentially on the same business model with slight directional differences. NK seems to be trying to build more consistency in their network whereas F9 loves to add destinations that they serve once a week. While that may not be important to you, if you like to non rev on company metal with your family my bet would be NK might be less stressful.