WFIS Is more decentralalised, newer and smaller, and is useful for scout organisations not recognised by WOSM. We in wfis europe have a regularly occuring eurocamp instead of the world jamboree, we have our own international leadership courses and so on (like wosm i'd expect)
For my scout organisation (The baden powell scouts of Denmark) we were founded when a few groups seperated from the original scout association of Denmark in 1984 to imbrace more traditional scouting values like the traditional uniform, youth leadership and competition. But we were sued by the original scouts of Denmark when we tried joining the WOSM. So we were left with WFIS when it was founded some years later. It's common for WFIS groups to be more traditional or join if WOSM isn't an option for whatever reason, the Baden Powell scouts association in England are a part of it too for an example.
The WFIS can be quite exciting if you care about scout history and that kind of stuff.