When no cell service the phone has no internal data available. (NAVIGON app was the exception with download of all states available, but has been discontinued.) Dedicated GPS has all of the map data internal but must be updated from time to time. It works all the time and does not require a cell signal.
As to the phone, google maps was significantly superior to Apple maps, but recently has improved ten fold and is now pretty much an equal, and occasionally better than google maps.
Having both dedicated gps and phone maps is ideal.