I'm with those who have said bring both, the dedicated motorcycle GPS units are tough, waterproof, glove operable, and reliably sunlight readable. The smartphone is a great backup should the GPS fail or should the maps you have on it be unreliable in some way.

Not many smartphones are waterproof out of the box, the ones which are not are a pest - many will overheat when put in a waterproof case and left switched on. They're also not very tough, particularly the USB ports - charging on the go will be essential and rough terrain (and vibration, particularly on a thumper) will destroy your USB port eventually. Then you can't charge and you are lost. This only phone I know which gets around these problems is the Sony Xperia range, which is waterproof and can be used with a magnetic charger on the side. The mag charger falls off sometimes but that's a damn sight better than a broken USB.

Personally I consider my Garmin to be primary navigation, my smartphone has the same way points and one or more offline map loaded into it for the appropriate areas. If the GPS died I would be comfortable using the phone, but I would replace the GPS once I got home - it is that much better when on the bike.

The phone is better for finding services, helping work out where you are and for using alongside paper maps, which you should also bring. One unlucky off could kill both phone and GPS...

__________________
[ Tim | History - NW Italy/French Rivera, Swiss Alps, Morocco | 2016 - Greece > Albania > Macedonia > Kosovo > Montenegro > Bosnia > Slovenia > Austria ]