I owned a Kemper for years while also owning an XL+ and AX8 at the same time; and now I just own the AF3.
The Kemper is good for what it is intended. If you have various amp/cab/mic/guitar set-ups that you like, and that's all you ever intend to use, you can profile all of your set-up/setting combinations and have them all in one portable device. Profiles are snapshots of a specific set-up and amp settings, however, which will require you to profile all of the amp settings you need for each particular set-up. This is why purchased profiles include many profiles with slight variations in settings (usually drive amount).
You can make adjustments to profiles with the knobs on the Kemper but, as pointed out earlier, they are general in nature vice programmed to react as they would on the actual amp you are profiling. Very minor tweaks won't impact it too much, but profiling a clean sound and turning up the drive knob on the Kemper, for instance, won't result in the same sound as capturing a separate profile with the real amp's drive knob turned up, if that makes sense. That applies to all knobs on the Kemper that relate to amp adjustments.
I purchased the Axe III last year and sold my XL+, AX8, and Kemper. IMO, the AF3 is better overall in terms of sound, feel, and flexibility in creating tones; the effects are much better than the Kemper; more I/O options; there's a computer editor that make things easier; and the USB recording capability is as easy as it gets for my purposes. You get a ton of cabs pre-loaded, and there are a ton more available for purchase. Updates are frequent and always bring something more to the AF3, and the community here is extremely helpful in offering advice and answering questions.
If you're worried about having to tweak sounds endlessly on the AF3, I think everyone would agree it isn't necessary. In fact, quite the opposite. Yes, there are a lot of advanced parameters available, which adds to the unit's flexibility, but they are not required to get where you want to go. If you're looking for a "done for you" option, there are quality preset packs available. Austin Buddy's Naked Amp Pack is the one that comes to mind right away for straight amp/cab sounds. Plus, in terms of time spent, I found I spent more time wading through profiles trying to find one I liked than the time it takes to dial in what I want on the AF3.
In terms of profiling your amps/cabs, you just have to consider how much equipment you'll need, if any, to create solid profiles along with the time will it take to learn how to profile in order to get the results you want? Just downloading some profiles from the Kemper community will tell you that it's not a slam dunk process. Then again, you may have the background and equipment that makes it easy to get the results you want in a short time.
Not bashing the Kemper. I had mine for 6 years and played it a lot. As I said, it does what it is intended to do, and does it well. It just depends on your needs. If all you want to do is profile your amps/cabs for recording and live use, then it may fit the bill. If you want to have the flexibility in creating solid tones, recording, I/O options, and playing live the AF3 provides, then that may be the route you want to go. For me, when I played through the AF3, I had found my one-stop shop for all of my guitar needs; especially knowing the unit will just continue to get better. Just my experience with the two units and some information on why I chose one over the other.
Hope that helps.