The maps in both the MapSource installer and the IMG file are the same. The MapSource installer is for users whose GPSr comes with a MapSource product which allows them to view and upload the maps from the PC. The installer just copies all the (IMG, TDB and TYP) files into C:\Garmin \mapname folder and creates the necessary registry entries in order for MapSource to show the maps.
The IMG file version are for those GPSr that don't include MapSource like the Nuvi, StreetPilot or Que. So, depending on which model of GPSr you own, you only need to download either the MapSource installer or IMG version of the map.