I own both. I’m the sort that always wants a back-up. I didn’t want two of the same. V8guitar is spot-on about the powered unit. If you buy a KPA, get the powered version.
Of course there are pros and cons for both, they take a different approach. That said, both are great. At this moment, there isn’t an Axe-FX III foot-controller - but it will ship in autumn. There isn’t a software editor from Kemper for the KPA. Yes, differences with fx. But I don’t use them much. I can get a cranked amp sound, by crankin the profiled/modeled “amp”.
Both devices sound terrific. Really, terrific. For what it’s worth, I run both using the same strategy: I use their main outputs to FOH, usually using the Ownhammer JBL D120 IR. Also for both units, I send a monitor-out to a SS amplifier. In the case of the KPA, it’s the integral SS Class D amp. For the Fractal, I’m using a Crown. For both, I use a Z-Best 2x12 cabinet loaded with two JBL D120 speakers. Therefore, except for the different SS amps, my comparison is apples-to-apples.
Bottom line: I love the Kemper KPA and I love the Fractal Axe-FX III. I’m a die-hard amp guy, and have a roomful of amps purchased originally, for next to nothing (which is why I first bought the KPA). Both Kemper and Fractal units sound great night-after-night. On the other hand, my amps have always sounded different night-after-night, presumably affected by the barometric pressure, lol?