Kemper, Helix, Axe-FX XL+ - some thoughts having owned/used all

There are things to like about all three.
Any of them can be used to achieve great sounding live/recorded guitar and bass.
 
Build quality:
I'd rank this pretty even.
All units are sturdy.  You'd expect this given the price.
That said, I'd take care if transporting to gigs (invest in quality gig-bag or case).
 
Form factor:
One big reason I decided to pursue the Kemper/Helix/Axe-Fx was to (significantly) reduce cartage to gigs.
Out of all three, I prefer the Kemper "toaster" form-factor.  It sits nicely on an amp-stand... and it's small/light.
The Helix is great if you're looking for an all-in-one "foot-pedal" unit.
Axe-Fx is a 2U rackmount unit that's 14.25" deep.  My least favorite of the three.
The Axe-Fx has an internal fan.  It's not loud, but it is audible (louder than my DAW).
 
User Interface:
Helix wins big time with it's UI.
Super easy to navigate/edit and assign controls.  Never needed to refer to the manual.
Kemper's user interface is my second favorite of the two.  
Though the "toaster" looks odd at first glance, but it's easy to navigate/operate.  
There's a dedicated button/knob for many functions... and nothing is buried deep in menus.
The Axe-FX XL+ is the deepest of the three units.  
With a relatively small screen and physical controls confined to a 2U face-plate, the Axe-Edit software is almost mandatory.
With so many options available, it's hard to distill the sheer amount of options to a simple UI.
I've spent the least amount of time with the Axe-FX XL+ (and I'm sure that's a factor), but I find it the most tedious to edit.
 
Realism:
With well-captured Profiles, the Kemper sounds virtually identical to the source (mic'd amp/cab).
Of the three, the Kemper sounds the most authentic to my ears.
The tone and dynamic response is exceptionally good (provided you start with great Profiles).
The Axe-FX XL+ with latest "Quantum" firmware is a close second.  
You can achieve well-balanced responsive tones that respond well to dynamics and guitar volume knob changes.
Though Helix certainly sounds good, to my ears it wasn't quite as authentic sounding.
 
Flexibility:
Here's where the Axe-FX XL+ excels.
You've got lots of DSP processing muscle (most of the three)... with four tier grid to layout the DSP "blocks".
You've got patches, scenes, and X/Y configurations to change guitar sound/tone.
Helix is a close second with similar grid layout.  
Helix lacks scenes and X/Y configurations, but the pedal/controller configuration is extremely flexible (can achieve similar results).
Kemper is the least flexible of the three units.  You basically have a "snap-shot" of a mic'd amp/cab.
You have a set of parameters for tweaking the resultant "snap-shot".
 
Conclusion:
In this short post, I've glossed over a lot of details.
Kemper, Helix, and Axe-FX are all incredible/useful tools.
After programming a patch for piezo equipped guitar, I decided to keep the Axe-FX XL+.
Using tone-matching (frequency or EQ matching - not to be confused with Profiling in the Kemper), I was able to program a patch that sounds ridiculously close to a real acoustic guitar.  
For crunch sounds, I think I sacrificed a *tiny* morsel of realism (compared to the Kemper) for loads of flexibility.
I've programmed a handful of different crunch tones (each very good)... and barely scratched the surface.
In just a couple minutes, I programmed an Ampeg SVT amp/cab along with a parallel high-passed distortion (a la Chris Squire/Geddy Lee)... that sounds great.
 
You can't make a bad decision with any of the three.
For me... right now... the flexibility of the Axe-Fx XL+ is hitting the spot.
 
 

Thanks Jim
I'd almost decided I didn't need any of the three; now.....
 
T

You guys are killin' me!
Its murder I say, MURDER!
Someone dial 911...

Thanks for taking the time to post.