Having owned or own all 3 of the most popular and talked about (Kemper, AX8 & Helix) I can really give you some unbiased raw opinion. Each has it's strength's. Since the OP is only interested in AX vs Helix I will give you my $.02 on those two. If you are a studio/audiophile and want to spend loads of time tweaking to find YOUR perfect tone and want loads of options available to you the AX8 may be for you. Keep in mind there is no recording interface and no headphone jack. Fractal tried to make the AX8 a grab and go for live gigging. If you want "ease" of use for live playing and "ease" of use for tone shaping, the Helix is far more intuitive than the AX8. I think Fractal had an A+ idea for the AX8 (live gigging floor mount tool) but a C- for execution for what it was intended for.
Helix Pluses (vs AX8) - Ease of use, selection of interface options and tone shaping. Large Color Screen, Nicely lit and organized display, Headphone jack, recording interface options, expression pedal included.
Helix Minuses (vs AX8) - Much smaller Amp selection of models available, Amp, effect and Cab Models are slightly less impressive as the AX but difference is truly insignificant, Line 6 reputation not as strong as Fractal
AX8 Pluses (vs Helix) - Better effects and Amp Models (after lots of tweaking), Much larger selection of Amps, Better Sounding Standard Cabinets, Fractal's positive reputation for modeled tone and firmware updates. More stable and useable desktop interface. U.S designed and built.
AX8 Minuses (vs Helix) - Primitive and small looking screen (monochrome), complicated layout, not easily understood out of box, no expression pedal included, not allowed to run 2 simultaneous amps. Not easily recognized changes on pedal in live settings, amps and footswitch changes due to poor design and layout. No headphone jack.
Both units are solidly built. Again, I think those guys/gals who are looking for the absolute tonal best option in a Modeler and are willing to spend loads of time tweaking and learning Fractal is for you. If you really want something that is user friendly and has much more "fun" factor but not quite the tonal quality (95%+ as good) then Helix is for you.
Again, this is my opinion. I never owned a Fractal AX rack system so my learning curve was much harder on AX8. The AX8 gave me some awesome tone and effects. No complaints in tone department. However, the tweaking options required to find my tone at times left me exhausted. I also found the screen and footpedal hard to read and not very intuitive. Where as the Helix has a large color screen and multi colored switching options to easily recognize your effects status and location from a standing position.
In balance, both of these and the Kemper are the best of this generation. Can't go wrong with either one of these.