The Garmin maps have generally quite good coverage - it's consistent, although there are still a lot of paths and PoIs missing.
Openstreetmap is excellent in some areas, far from excellent in others; in particular, tracks and paths are likely to be less well mapped to date. However, they cary a lot, region by region - you need to check the areas you're interested in.
There is a very big benefit to OpenStreetMaps, and that is - all the mapping data (trails, streets, bathroom stops) is editable by users. I've added greenways, entire park systems and roads to OSM. Very easy to do.
