For me, I want a GPS that I can use both on and off the bike, so it needs to be hand portable (walking) and water proof (riding in the rain). A tablet would not suit me.
'Free' maps tend to be what people use - especially on a world trip where the costs of maps tend to be high for the amount of use you would have. OSM tends to be the source of these free maps in various forms - on a GPS, a 'smart' phone, a tablet. While you might see the app called 'maps.me' for example .. the base data is OSM, only the way in which it is presented and the amount of data used changes from one app to another.
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