I only skimmed the differences between the two. There are many projects compatible with 3 that are not compatible with 2. Even some OSM projects are version 3 of GNU.
I wanted to use the API pf OSM's Open Railway Map https://wiki.openstreetmap.org/wiki/OpenRailwayMap/API and use the https://www.openrailwaymap.org/ for more accurate railways. I know there already are railroads shown, however this is for different purpose. We already have aircrafts, space crafts, cars, and even ships on FG. However we do not have a train. to add train simulation to FG we would need a more accurate layout of the rail network.
Unfortunately, OSM's Open Railway Map is GNU GPL v3. https://wiki.openstreetmap.org/wiki/Ope ... p#Software