The internal or external just refers to the GPS receiver/antenna as being inside the unit or as a separate unit apart from the graph itself.
With the external GPS if something goes wrong with the GPS unit one could still keep and use the graph while the GPS was being repaired. With internal GPS if something need fixed with the GPS the whole unit goes in and you don't have use of your graph during that time.
With the external GPS I have seen where the receiver module was stuck on the back end of the boat while you are in the front of the boat using your graph and GPS. I always felt like the receiver needs to be right up there with the graph while hovering over brush piles you are working.
“There is no difference between communism and socialism, except in the means of achieving the same ultimate end: communism proposes to enslave men by force, socialism—by vote. It is merely the difference between murder and suicide.” Ayn Rand