IMHO it's more about the overall feel under the fingers where the AFX III outclasses the AX8. Of course it sounds better, more 'alive', 'beefier' and 'open' but both units are in the same ballpark tonewise, it's not like those comparison videos where you put a Helix vs the AFX using the same settings and both units sounds really different from each other.
Even though I'm actually selling my AX8 (If it sells, good... money will be spent on the FM3, if not, I absolutely don't mind keeping it as a backup) still hoping for Cliff to put out the Ares port for the AX8 to narrow the gap between the two, but right now, I'd say it's absolutely worth the money to upgrade, specially given the price drop on the Axe.