For identical years, the map data in the two products is the same. City Navigator NT uses a more effective form of compression which allows more maps to be stored in a smaller space, but it does require newer units; all the units covered by this FAQ support NT, but double check with Garmin before purchasing.

If you're purchasing new and don't have to worry about compatibility/shared unlock codes with an older GPS, there is no reason to NOT get the NT product.

As perspective, the entire US almost exactly fills a 1GB card with City Navigator NT.

24K maps are available from Garmin, but only as part of their National Parks product.

No. Those programs may be able to transfer waypoints, tracks, or routes to and from the unit (the details will vary for each) but they do not send maps.

If you live in an area where Mapsource is available and you place any value on your time, it would be hard to encourage you to roll your own. However, if you're suitablly determined, you can investigate the 'map_authors' yahoogroup and find tutorials for making custom Topo maps for your Garmin. There are some canned maps available on CGPSMapper.

When you upload maps to the unit from withing Mapsource, the maps already on the unit are erased.

When you upload maps to the unit from withing Mapsource, the maps already on the unit are erased.

Garmin has a site for reporting errors in Mapsource. If the correction is accepted, it usually takes one or two subsequent releases of Mapsource - typically annually - before you see the correction implemented in a released product.