You can check our main intercom compare table(include M1-S Plus and FX-8 ) here:
The biggest difference is M1-S Plus had a music share function,( sharing music or other audio files with rider or passenger) , M1-S Plus also is the most recent intercom(UPDATE version of M1-S pro).
FX8 had a Pop-Up antenna (30% Stronger Signal.)
It depends on your needs 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 people simultaneous call ( 6 people have the best performance)