The biggest advantages to using a GPS device are:
-You do not need a cellphone signal to load maps, and it will not use up any data to use
-Better accuracy, and faster at getting a signal and updating
-The GPS is more rugged, if you drop it into a mud puddle, just wash it off, and it will be fine (try doing that to a cellphone! )
I use both, my cellphone in the city, and my gps elsewhere. I own the eTrex 20. I love it. The only thing is that it doens't have a touch screen, but I like it better thay way. Might I recommend the Garmin Oregon series?