That map is routable and the date of the last update is show just below the slippy map.
Dave Hansen grabs the tiles (the squares as shown on the slippy map in the above link) and merges them into a complete map of the USA. So you can choose which source for the US you want. For large areas Dave’s map is handy.
Don’t know, but I’d think OSM will do pretty good in highly populated areas compared to the commercial products.
I think it depends mostly on the main usage. A car GPS requires other things that a handheld for cycling or hiking. A GPSmap 60 or 62 will be top for the latter.