I have used both these editions, and honestly I don't see any difference in stability whatsoever. Both do everything I need without issue.
So my question is. Is the LTS edition more stable than the user edition, or is it just that it is supported longer?

There is no practical reason why one would ever want to use the LTS editions. The single difference is that the LTS edition uses Plasma LTS releases, whereas the regular edition uses whatever is the latest stable release. Meaning the LTS editions doesn't get new features in Plasma. This kind of runs counter to the idea of neon though, as is evident by the fact that *everything else* still gets new features. It's only Plasma that is stuck in the 90's, so to speak.

It's not more or less stable, it's more reliable though. Point being, features sometimes cause new bugs, so not adding features reduces the chance of introducing new bugs. On the other hand some bugs cannot be fixed without adding new features, so they won't get fixed for a very long time.
The way to look at it is as long-term-reliablity really. You can rely on the LTS edition's Plasma to have bugs x and y, but it's not very likely that there would be a new bug z. The regular edition may, however, have fixed bugs x and y but in the process introduced a new bug z.

All that said, keep in mind all this is limited to Plasma. So, if you find yourself thinking that you want this type of reliability you'd probably be better off using one of the popular Linux distributions that offer this for the entire stack (kernel, plasma, applications...) such as opensuse leap or debian stable.

As far as neon is concerned, the regular User Edition is your go-to option. It's what gets tested the most, is used by most, and is closest to what the KDE developers will be able to fix bugs in.

Thank you for your comprehensive answer.
At present I am using the user edition, so I think that's where I will stick.
Just another related thing. When Plasma 5.10 is released, will the user edition be automatically updated to that, or will a new iso install be needed?

New versions are delivered through regular updates

Thank You.

I must disagree a little! First of my personal computers are all encrypted, and all I've been using KDE neon as my main OS on all 12 + computers, work, home, several desktops, and laptops for over a year now. I experienced a major break after an upgrade on two of my desktops, and two of my laptops with the standard KDE neon. A Kernel panic! after an update, where I was not able to recover from it. Then I installed the LTS version, and never had any issues since then up to this point. So the LTS version is more stable, specially if your hard drive is encrypted.

+1. I believe there will be users who want the LTS directly from the developers. Like a nexus/pixel phone from google. One Plus 3 is very close to stock from a look and feel, however users buy nexus and Pixel because it comes from the source (Google).

Similarly I believe Neon User LTS has potential that we will realize in next few year.

I suppose this is all a matter of perspective. As was mentioned in post 2, if you're using the LTS, and everything just works, then it's unlikely new bugs will be introduced. However the opposite is also the case. If your using the 5.8.6 LTS, and have issues, then those issues will never be resolved.
Think I'll stick with the standard user edition, and see what happens. If show stopping bugs are introduced along the way, then I may think again. But for me the latest user edition is problem free.

Ive seen difference on LTS and User edition...my grub is still working after a sudden shutdown on LTS! haha...

@apachelogger Thanks for your explanation. I thought LTS was more sur and I use it,
but some translations are missing in french and maybe the regular edition would be better for this little kinds of inconvenients.
So, I think just a little change to do in /etc/apt/sources.list.d/neon.list :
deb http://archive.neon.kde.org/user/lts xenial main
deb-src http://archive.neon.kde.org/user/lts xenial main
but what exactly should I put in place ? just take off "/lts", putting "stable" ?I did not saw the information, excuses.
Then could you confirm I should just do
sudo aptitude update
sudo aptitude upgrade
sudo aptitude dist-upgrade

@vegangaro seems you are a bit iranical, I usualy apreciate that, but with my poor english I could miss something.
Is there anything you wanted to said that I should care about ?