You can check our main intercom compare table (including M1-S Plus and FX8) here:
https://www.fodsports.com/fodsports-helmet-intercom-headsets-comparison/
The biggest difference is M1-S Plus has a music share function,( sharing music or other audio files with rider or passenger), M1-S Plus also is one of the most recent intercoms (an updated version of M1-S pro).
Besdes, the latest version M1-S Plus also improved the intercom number ability to 10 riders. This makes sure it can support a larger group of riders.
FX8 had a Pop-Up antenna (30% Stronger Signal)
It depends on your needs while you are choosing among them.
If you want to enjoy music with your rider partner, you can choose M1-S Plus.
If you want to ger stronger signal intercom for a Longer distance, can choose FX8.
FX8, M1S Plus both support 8-10 people simultaneous intercom ( 6-8 people have the best performance)