I have both an FC-6 and an FC-12. In fact, I find that using both of them together is overkill for anything I’ve done so far with either my Axe-Fx III or my FM3. I typically split the two FCs between the two devices.
All else being equal, I prefer to use the FC-12 because it gives me one-button access to a lot more stuff, and it’s nice to minimize hold functions. But in practice, i take my FC-6 almost everywhere because it’s so damned transportable, and it gives me enough functionality to do what I need to do.
Where two FCs come in really handy is for bigger shows where you want to have two stage locations, or there’s a tech belowstage doing some of your switching. The ability to slave two FCs together is really handy for that. Otherwise, how you use them is very much a personal-preference thing. Remember you’ll probably want to do more in a show than just change presets and scenes.
Regarding your question about chaining an FC-6 and an FM3 to an Axe III, that depends on your definition of “chain.” You can make the FM3 send MIDI to the Axe, but you can’t directly link all three devices via FASLink.
