My Axe Fx III just came in last friday (woohoo!!!).
With my AX8 on Quantum 10.01 and the Fx III on Ares 11.01 I can tell there's a difference, definitely, but it's not 'abysmal' as some may think... after all it's not like back in the day when the original Axe Fx came in and kicked the POD to the mud in shame. Here you're talking about 2 products sharing the same core philosophy in modeling.
I've copied the settings from one preset, from the AX8 straight into the III (Same amp and 'legacy' cab) and the basic DNA is there, both are on the same ballpark. In a live setting, having one guitarist use the AX8 and the other use the III I bet more than one person will have problems telling one from the other.
Now, recording both... I can tell the III is definitely beefier and more '3D' sounding. Gels better in the overall mix... I'm really stoked and psyched to dive deeper into the ARES ecosystem. If you want to take the leap, I'd say DO A BACKFLIP.
Once again, having the III with me makes me more EAGER to try ARES once Cliff unleashes that sweet baby for the AX8 and compare it to the Fx III, but man, the AX8 on Quantum 10.01 can hold it's own against the Big Boy.
Just my two cents...