As others have noted, unless you have paid for a specific "GPS" app that downloads maps to the phone (and takes up BIG chunks of it's memory), you can run into problems when in "no coverage" areas. Suddenly, Google will not load a map of the area, and you're navigation is dead.
Cell phones have "budget" GPS chips - they're accuracy is enhanced by information they get from cell towers and/or wifi. Without that, they can be pretty poor at accurately telling you where you are.
Dedicated GPS systems have much better satellite reception, and can then always be much more dependable.
Routing algorithms for either can be hit-or-miss, so neither can be better than the other - they can both get you there, but it may be a pretty exciting route. Says the guy who just got back from trying to navigate a big truck for the first time into Brooklyn, NY!
