Hi Peter,
I see this question lingering for several days without any replies. I don’t think I have direct answers for you, but I hope to give you something more to think about.
I am a fairly new learner. I picked up my first guitar just over a year ago, and never tried any other instruments, so everything is pretty new to me. I was pondering the same questions for similar reasons. I initially bought a Spark modeling amp for practice after giving a try to minimal alternatives. About 4 weeks ago, I bought a Helix.
My reasons for the Helix were:
- I really wanted a looper
- I really wanted some expression pedal control
- I wanted to hear various sounds and be more free to route the different elements of a guitar chain that the Spark didn’t support
- I cannot clutter my wife’s living area with a pile of pedals
- I am fairly nerdy
There’s more, but those are the bulk of it. 
I examined the products from most of the major name brands, Boss, Line 6, Kemper, etc. I listened to a lot of review videos and eventually I discovered a You Tube video series from Leo Gibson. I like his test methods and side-by-side comparisons. I also looked at the electrical specs - the published information is quite limited, however.
I chose the Helix LT for how it sounded and for the large memory space to save configurations, the large number of buttons, and number of models - and it had the expression pedal.
I really like having bought it. I still have some concern that Line 6 will put out a unit with better electrical specs, but I am becoming less concerned that I need them. It is just my engineering self that would want something better even if I cannot hear it.
I use the Spark for the speaker. I created a flat and clean profile on the Spark, then measured it with a tool called REW. I measured the flatness I could achieve using the Spark’s features, then created an IR from REW that finishes the flattening that I use in each Helix configuration. This works fairly well for my home.
The only feature I miss so far is not having two guitar inputs. I have two guitars and like to avoid plugging and unplugging them. I currently use the return inputs instead of the guitar inputs since the LT only has one guitar input. The full feature Helix has 2 guitar inputs, but cost is nearly double, and I didn’t want to think about failure of the scribble strips.
I have found myself fiddling with it far more than I expected. This means I am not really getting in a practice, just running a few chords or riffs then making an adjustment, and doing them again. It is something, but not really practice!
If you have questions I may be able to answer, I’ll tell you what I can!