So I just bought the Helix LT. I've been using a Kemper live for the past 4 years, so curiosity got the better of me, and I had to try the Line 6. (I was using an HD500 before the Kemper, so I'm familiar with Line 6)
My rig goes straight into FOH, using the on stage monitors for reference.
My initial thoughts, the Kemper has many more options of tones available, some good, some bad, some unusable. I downloaded a 5150iii profile for the basis of my sound, and its covers everything nicely.
For the Helix LT, same thing, some good, some bad, some unusable. However, I was happy to find the Marshall 2204 patch in the presets, and with minimal tweaking, came up with a good core sound for building patches.
It feels to me like the Helix has a little more response and feel than the Kemper. Maybe a little fizzy when you start adding gain for lead, but I'm very pleased with the punch and tightness of the dry rhythm sound, and you can get some great dynamics out of it by playing harder or softer.
And the Cleans on the Helix are much more pleasing to my ears.
I really don't need as much amp as the Kemper offers, and I think the Helix LT will fill the role nicely of very usable and workable tones for playing in a live cover band every weekend. And new toys are always fun.