I briefly tested the new beta 4 firmware on axe fx III. Many amp sims sound and feel different to current fm3 firmware to me.
But I believe that's temporary, per usual, and dependent on firmware advances the core of which fm3 is most likely to get.
Once fm3 gets whatever the final release of Cygnus X-2 is, considering all we've seen so far, I believe it'll sound more or less the same to axe fx running similar firmware (even though by that point axe fx may have Cygnus X3 Lel).
I say "more or less" because latency will be most likely be different -- whether this is perceptible while playing or not, I don't know -- and there's a few extra controls on axe fx 3 amp sims. I don't typically care about these, so it's not an issue for me.
I've made tests where, considering similar firmware, I saw no meaningful or even perceptible differences between fm3 and axe fx 3... Not for any of my tones at least, either for feel or recorded. It would have been impossible for me to tell you which is which even in a blind feel test.
And usually I'm quite perceptive when it comes to stuff like, I would say.