You're over analyzing it.
I ran the Helix Floor in freakin' 7CM into two amps in stereo, with each amp having pre/post fx. That still was totally fine for both tone and latency. Obviously a totally insane rig to use, but I did it just to try it out.
When the QC released 3 years ago, I owned the Helix Floor, QC and FM3 at the same time. I extensively compared them back to back.
The most important differences are going to be:
- Workflow. At the time the QC was the best onboard UI, the Helix a good in-betweener and the FM3. Axe-Edit had the best computer editor, QC had none back then, and HX Edit is "meh, good enough."
- Effects quality. Not amp/cab sim quality, effects. FM3 > Helix > QC for me.
- Form factor and footswitching. QC is a bit cramped for stomping, but a great desktop unit. Helix Floor has everything you need but is hideously large for my tastes. FM3's 3 footswitches are just not enough IMO for all the features it has, and it's an inelegant, somewhat chonky box that doesn't go that nicely on a pedalboard or desk.
Latency, conversion quality...these are totally irrelevant factors IMO. The top units are all good enough in this regard.
For amp sims, consider if it's more important to you to have good tones, or if you are a stickler about the accuracy of things. This will determine if you are ok with e.g Boss, Hotone, Kemper etc, or if you should go for say Fractal. Getting tones that sound good and are fun to play is easier than ever.
For effects, Fractal rules the roost, but Helix provides a more pedalboard style experience. QC is alright, but not going to blow your socks off.
I sold the QC because I did not trust NDSP's roadmap. It was the right move back then. I sold the Helix Floor mainly because I wanted a desktop friendly unit, and the Helix was definitely not it, while the HX Stomp is too gimped in onboard UI. So FM3 was the compromise, even though I don't like using it without the editor. But I'd still recommend Helix any day for someone looking for a do-it-all floor unit.
IMO the Tone Master Pro is atm where the QC was at release. Unknown roadmap and not a mature product.
If you just want something different to try, the Hotone Ampero 2 Stomp or Stage is pretty inexpensive and is kind of like a "QC meets HX Stomp" in many ways. The Stage has the capture feature which might satisfy you for amp sounds if you have amps to capture, or maybe you can capture your IRX to slap it into the unit. A2 Stomp is likely to get the capture feature next year.
I think you just need to buy a unit or two and just try them for yourself. This is easier if they are something sold by a major retailer and don't cost $1500+ a pop since it's easier to afford a few and return the one(s) you don't like.