I just returned from the South Carolina flood area where I spent two weeks as an American Red Cross damage assessor. I took my Garmin and my smart phone.

Started off using the Garmin and within a few days, used the phone exclusively to find my way around the affected areas.

The phone allows voice programming, a time saver.
The phone provided more precise directions by updating position more often.
The phone assisted in finding routing around road and bridge closures (Waze). There were many as you may imagine.
Of course, I haven't received my bill yet so that may change my opinion.

All that said, I took the Garmin to Italy a couple of years ago. Bought the chip for it and all I can say is that it probably saved our marriage.