I've been using Garmin for over 20 years. You can't really beat them for the price. The standalone GPS devices like TomTom, Garmin, etc. will always be a better, more reliable, more accurate means to navigating a vehicle are yourself. They are designed with good GPS chips that have everything they need to navigate. Cellular phones don't have true GPS like the above mentioned devices. They instead rely heavily on supplemental location information derived from cellular tower locations.

Bottom line, if you are serious about navigation use a GPS unit by Garmin or some other reputable company, not a cellular phone. The iPhone and Android phones do a lot of things well, but navigation isn't one of them.