i have both kemper and axe FXII.
basically, on both units you are going to need time to get the sound what you want.
on the kemper it might be a long struggle of search through tons of profiles until you find what suits best for you.. and on the axe fxii its similar with the cabs/irs as the cab section in the axe fx really makes the difference.
i needed about 3months to get and find my sound with the kemper.. since then i now just use a handful of profile and its done.. no tweaking, just playing.
to be honest, the first couple of days was pretty hard with the kemper and i thought to return it to the shop. back then the stock profiles didnt sound very great and there were not much profiles packs either, today its a different story, you have lasses profiles, olas and the free amp factory profiles are a great starting point where you fast can get awesome tones out of it.
Of course the kemper has lots of parameters you can tweak, but as there is no software for computer to control it yet, so you all have to do it on the kemper itself.
on my axe fx ii i also needed about 2-3 months to got my sound going, on the first day i had instant access with rebuilding the sound of my splawn quickrod amp in the Axe FX, the sound was quite spot on.. in like 3minutes of work, but then the "tweaking process" starts and that really can take up all your time, instead of using the time and play guitar.
There are also great Stock presets and freeware presets like the "fremen-package" which gives you a good starting point.
Both sounds and play pretty well, its just a matter of philosphy.
with the axe fx you also can get unnatural sounds, like super-bright and clean stuff, or acoustic simulations, the kemper can sound more real and raw, but in my opionion, real and raw is not always what you want, for me at least.
I really like building my own sounds from scratch up, so of course that fits better with axe FX, but the kemper has also so much nice tones, that i cant let it go and use it on a daily basis.
If you play live and want to use this unit for live or rehearsal too, i suggest the axe fx if you are going to switch alot through profiles and different sounds, if you just use one sound you can ignore this one.
The Scenes Function on AXE FXII is really awesome and you can set it up that you really have ZERO lag or soundrop during the switch from one scene to another, and the change reacts quite instant with no noticebel lag.
Kemper does have the performance mode, to switch from one profile to another works fine, but not as swift as on the axe fx II.
The Interface of the Kemper is pretty intuative and very easy to use, you learn very fast how it works and you can set the parameters like Gain, EQ, etc. directly on the interface.
the Axe FXII Interface is in my opinion far less intutive, you have to go through menues to look for your paremeters. but you do have an PC/MAC Editor that you can use which is far more comfortable to use, at least for me, there is no PC/MAC Editor for the Kemper yet. but its not really needed in my opinion, only a librarian tool would be nice to have.
dear kemper and axe fx ii users, please dont take that the wrong way.. i tried to write down my personal opinions as honest as possible. both units sounds great and feels great, they are just different and each has its pro and cons.. just that the other unit can do something slighly better doesnt mean the other unit is not good enough.
cannot say anything about the axe fx ultra,as i have never played one,