Each GPS manufacture has different (and incompatable - at least at the user level) software and data. If you already have teh GPS then you are stuck with what is avalible for it .. unless you buy another one.
So look at what each manufacture (or other/s with compatable stuff for that manufacture) offers in the way of maps. See what the data is like, and how much $.
Back when I was picking - only Garmin had a 'World Map' .. so that is what I got. That world map is a little better than a large paper map of the world .. when you zoom in on say New York ... you only get rought lines of where very major roads are .. and they can be in error by a few miles ..
Things are getting better thanks to nonGarmin sources - SmellyBikers WanderLust map for instance ... and it will get better over time .. But if you want good data for North America you'd be better off buying map data for just that area .. Most America people chose Garmin. The altertnative is Margellan (sp?) ... Don't know what the coverage of things like 'TomTom' is like for South America... would not expect much from the at this stage.
Regards Frank Warner
motorcycles BMW R80 G/S 1981, BMW K11LT 1993, BMW K75 G/S