I switched to Helix for a couple reasons, one being the lack of modern bass heads. I was playing a lot of modern jazz bass (fretted and fretless) and the fractal support for that isn't as good as the line6. I also liked the form factor with the Stomp and how friendly it was to sit on a pedalboard.
I loved the modern bass amps on the Helix. I was able to get a sound close to those of modern jazz bassists janek gwizdala and tony grey.
I also really liked the reverbs and delays on the stomp.
However, the tube amp modeling and cabs always bothered me on the helix. I was never able to get a direct tone I liked when recording. When i practice, i use a “non-IR” output on the Helix/Fractal to goto a power amp and then into a pair of mojotone cabs with mesa blackshadow speakers. I began mic'ing the guitar cab and mixing that with the direct sound in order to get a tone I liked.
Due to neighbors with barking dogs and leaf blower fetishes, this limited the ability to record and I spent hours and hours trying to get a direct sound I liked.
On a whim, I found a good deal on a fractal FM3 Mk II Turbo and I bought it.
Within a couple hours of plugging it in, I was getting better and more realistic direct sounds than 1.5 years of mucking around with the helix.
Prior to getting the fractal again, I had a couple brief flirtations with sample-based modelers.
I bought a quad cortext and found that its reverbs were toy-like and had some issues with extremely low output on the headphone jack and inability to use the effect send as an output to my power amp. It did have a great and more realistic (than the Helix) fender tone but I sent it back a day after getting it.
I also bought a tonex but it was a complete failure. First of all, it installs itself as the default audio device but cannot be used as an audio device out of the box so I spent a frustrating ½ hour trying to figure out why I had lost audio on my computer. Then, I discovered that it's installation software had disabled the main outputs on my presonus 68/C device. But then, I discovered that it had no ability to configure one output with and one without IR - which is a requirement in my monitoring situation. And in order to use it to configure sounds, you have to disable it's playback by going deep into the setup menu (this enables its use as an audio interface). Then you have to edit sounds on a virtual VST on the computer, save, download to the device and then switch the setup menu back from audio device to “live” - which is playback. And on top of all that, it's software is a complete UX disaster.
So it got returned also,
The fractal sounds much more natural than the helix. Much more amp-like feel whether you are listening direct or going through guitar speakers. It sounds like an amp, feels like an amp and reacts to pedals like an amp. On top of that, the effects are VERY GOOD. Head and shoulders better than tonex, quad cortex and kemper although I like the kemper a lot.
And speaking of pedals, the fractal implementations of time-based effects like reverb, delay, vibrato, modulation, rotary speakers are vastly superior to anyone else although helix and kemper have very good implementations.
When it comes to stomps - overdrives, wahs, auto-wahs, envelope filters, nothing compares to the fractal. I have several expensive dumble style pedals and you'd be hard-pressed to tell whether the drive you are hearing is from a virtual one or the real pedal.
Fractal has shown how powerful an algorhytmic modeler can be. There is no dependence on someone capturing the amp and what gain settings they used, what tone settings they used, what cab they used, etc., etc.
The only thing missing with fractal is that they have no modern bass heads. So if you play modern electric bass in the style of tony grey, janek gwizdala, hadrien feraud, matt garrison, you may find it frustrating. You will most certainly have to buy 3rd party IRs for the cabs and like me, you'll have to take an existing amp or preamp and get into the bowels of the editor and modify it to work for bass. Since I don't play bass much anymore, it's not that important to me right now. This is the one area the helix beats fractal. The bass amps are much better.
But for literally everything else, the fractal reigns supreme.