I donít think thereís a clear winner in this discussion. Dedicated motorcycle GPS are easier to use, glove friendly, weather resistant, etc., but are ridiculously expensive for what they are (IMO).
Iíve used a Garmin Zumo, sold it and used my iPhone, bought an iPad mini and used that, sold that and now Iím back to a Zumo.
As far as iPhone apps, I like Co-Pilot. However it cannot share routes in any easy way, and you canít plan a route on a laptop and transfer to the app. Glove friendly also matters - even with ďiPhone friendlyĒ gloves the iPhone screens are hard to use while riding.
So the cheapest method is a smartphone (if you have one already, or by buying a cheap older one) and an app. But there are drawbacks. A Zumo is much more convenient, but thereís a large premium to pay for that convenience.