Basically they are the same.
Think of World as DCS A-10C with the Map Only, because that's what it is.
Now the map is free and you buy the aircraft that go in it.
If you only have the A-10C module the result is an updated version of the original game. Same with the KA-50.
The big difference is that if you own multiple aircraft you don't have to install the entire game each time, and the developers don't have to update each title as well.
The model is the same as the FS series and Xplane.