I have had all three. I currently have a Kemper and the Helix.
I had been playing a Kemper for the last year and really enjoyed it. I picked-up a Splawn Street Rod and wanted to give the Helix a spin using the 4cm knowing I could return it. Being it was Line 6 and I hated the HD series, I truly expected to return it after a week or two. However, I knew a day later it would be staying for a while and I have zero qualms regarding flipping gear.
I was shocked. It sounds great! The modded Marshall (2204) is as fun to play and feels as good as anything I have used in the Kemper or the Axe Fx. I have enjoyed the Vox models and the SLO. I also really like the Shiva model. The pedal selection is cool and I have zero issues with the reverbs (which some say are completely lacking).
To be honest, I have been playing it exclusively through my Fryette Powerstation and never even hooked it up to the Street Rod. The foot controller is easy to use and the scribble strips are really cool. Adding your stomps into the four loops is easy as pie.
The full screen isn't just a nice to have...it's an amazing feature that makes you realize the Kemper and Axe both missed the boat here. The units workflow is intuitive, the joystick is an easy way to navigate the fields and the forums have tons of great threads and videos. It's also pretty awesome to see the people at Line 6 responsible for the Helix weighing-in and adding to the discussion. They seem amazing.
I am 95% sure I will be selling the Kemper. Not because it isn't killer but it's really nice to have so much access to amps. Profiles are easy to find and there is a good community there to learn tips and tricks. It's also nice that they aren't updating the unit what felt like weekly with Fractal.
I used the Kemper for a few days last week to make sure I wasn't crazy. It's truly an amazing piece of gear but I didn't feel like I was missing out by having been playing the Helix the past couple of months.
I really enjoyed the Axe Fx (man, I bet I owned at least six or seven of them) but the community wasn't the same as what I have seen with the Line 6 crew and the epic 2500+ page thread there. And while Cliff if a genius, he speaks a bit above most of his users and it's tough to get a clear answer other than "user error". The editor is great but again, almost too deep for me. (I was several updates into Quantum when I sold my unit fwiw)
So it's currently the Helix for me. One of the the things I really like about it is the ease of use. Most major parameters are driven by knob twists. Editing on your computer is super easy. Personally, I have found not having really deep access to parameters is a relief.
Buying the Helix floor, you can get Helix Native for $99. This is a plug-in with the full enchilada.
I didn't expect it to be this way but I would suggest trying a Helix. GC carries them and I was able to use a 15% discount. If you hate it, hit me up for my Kemper, controller and pedals!
Hope this helps a bit.
