To elaborate on a big point someone mentioned, the speed of updates on the Axe-FX III is much faster, and that's what you want if you want the latest models and features as soon as they're available. The inventor, Cliff Chase, actively develops that unit directly. One update was the result of him waking up at 3 a.m. realizing how better to model the heating of a voice coil, if I recall correctly.
These updates can be very very fast, very often, though a few months can go by.
On the other hand, the FM9 and FM3 develop at a much slower rate. Not all updates will never ever go to those units, or even be possible on the FM3, i.e., due to the less powerful processor, but whatever has been updated on the Axe-FX III that can be moved will be ported over by other engineers.
It takes much longer, and you'll also have to join their their subgroup within our cult in to swear your blood oath to wish publicly at least once an hour on for the latest updates to be ported over. These are the rules.