My Garmin portable kicks the phone's butt in every way for navigation.
Remember the old saying "Jack of all trades, Master of none"? The Garmin does one thing, Navigation (yes it does music and bluetooth etc) and does it very well. I'm a Blackberry user and our recent trip to the Dragon was made much easier with our Garmin. It took a few minutes aquiring satellites when in the mountains, but worked all the time.