It's different in that it uses U.S. Census Bureau Tiger Data to generate maps as vector data(lines) . Maemo-Mapper downolads maps as images.

Maemo Mapper can download multiple map types from different servers(topo, roadmaps, etc...) While Roadmap installs pre-compiled maps based on Tiger Data. So no Internet connection is required to view maps or search for an address if the appropriate maps are installed. Both Roadmap and Maemo-Mapper require Internet access to download and use routes.
If you have broadband Internet access, or require non-roadmaps (i.e. topo maps) Maemo-Mapper may be more practical. As it uses information from Google maps, Open Street Mapsand other sources, the maps are more likely to be current. But you must have an Internet connection to download them. Roadmap maps are installable by State.

Hope this helps,
kernelpanic