I will buy an FM9 and stick with the FM3.
Our other guitarist got my AX8 after I got the FM3 (since we wanted to go "full digital" guitar-wise). After fiddling a while with the FM3 it turns out that
- I don't feel restricted with the CPU limitation(s),
- with my additional double switch I get enough switching options for 80% of my use cases,
- I really like the small form factor.
At the end of the day, the FM9 will be my "main work horse", but I use the FM3 as backup for me and the other guitarist, as well as my "I am traveling around"-option. If I would need to fly to a gig, I would probably stick with the FM3 as well, just because, as you said, size and weight are enforced and limited.