What is the difference between Interrupt on Change and External Interrupt on PICs
Look at this document which states:-


  Interrupt-on-change This feature is similar to the external interrupt facility , except that a port change
  interrupt will be triggered by any change (not just one type of
  transition) on any of the pins for which it is enabled. This makes it
  more flexible (being available on more pins), but also more difficult
  to deal with correctly.