Believe it or not *absolutely* nothing. One method calls the other. However because the latter is named "Changed" it can be used for data binding - so while I dont have any concrete proof, that's my theory on why both exist.
However, if you are doing any real work in either of these handlers, you may want to consider using the Layout event instead. Also if you're just using this event to call Invalidate or Refresh (to repaint when the size of the control changes), consider using SetStyle(ControlStyles.ResizeRedraw, true);