I've used both, among other units. For a while now just have an fm3.
The biggest "lift" (at least as a feature I'd use) in axe FX III is the EQ matching function. Not quite as many fractal users seem to care much about that, but a lot can be achieved that way. There's been cases where running EQ matching on the III would get me where I wanted quicker than FM3.
That said, I grew relatively adept at tweaking the fm3, pretty fast, and was quite confident in being able to replicate nearly any tone I'd imagine, that way. I replicated nearly all my favourite Kemper profiles without even shooting an IR (one was a challenge, eventually bridged by shooting IR). Spot on in every way? No! Close enough? Yes -- with an advantage for "feel" in most cases. And it was the same against real amps, minus perhaps the feel advantage, at least with the latest firmware at the time.
Long story short, having EQ matching for me personally does not quite justify going to the III -- never mind portability reasons.
But sure. That's the biggest upside the III has for me compared to fm3, hands down. It would certainly see the light of day if I had an axe fx 3. Not nearly as much as I once believed, but for sure can be very useful, especially in some cases.
Cab smoothing is not something I care about much at all.