Kubuntu is Ubuntu. So, two major updates a year, where the state is basically "frozen" in between. This gives everyone time to fix bugs, giving you a more stable experience.
Neon is rolling release on everything regarding KDE, but has a very stable base OS. Advantage: newest and hottest KDE stuff, but it maybe has some rough edges and doesn't provide you with the newest kernel, which usually isn't a big deal, but you might miss out on something.
Is there a reason why you want something Ubuntu based?
You also have a few other options:
Debian: older (still has KDE 5.27), but very stable and tested
Fedora KDE: similar to Kubuntu, but more vanilla and less shitty in my personal opinion (Snaps, etc.)
EndeavourOS: Arch based, beautiful customisations applied OOTB, sane defaults if you like Arch
Fedora Atomic KDE (Kinoite) or Aurora: the newest image based (immutable) variant. It's what I use and would recommend. Very reliable!
Maybe I was lucky but I didn't experience many bugs with Plasma 6. One or two minor ones that were fixed quickly.
What I like about it is that Plasma with Wayland is now mostly bearable with an Nvidia card. It just needs Tumbleweed to update the Nvidia drivers and I think it will there. To the point I am considering trying EndeavourOS which I'm sure already has the latest drivers.
I have now. It looks quite interesting too. I'm still trying to convince myself that out of date Nvidia drivers are a reason to move, guess I'll see if we get the 560 drivers.
Ah okay. That makes sense. I have been using Fedora Kinoite on my test laptop and really enjoy Plasma 6 but I need it Ubuntu based so I can use Golden Cheetah with Ant+ devices. GC's documentation focuses on Ubuntu based systems and I got it working on Linux Mint, my main laptop. I just like Plasma more than Cinnamon and am slowly changing all my systems over.
If the software you have to run is specifically designed for one distro (e.g. something that's only in the AUR, or written for Debian) you can use Distrobox.
This creates a small, lightweight container that allows you to run any software from all distros on your host.
I, for example, use Fedora Atomic, and I mostly use an Arch container that's fully customized for me, including having the AUR enabled.
If you liked Kinoite, then you can still consider it and run your stuff via Distrobox
The program itself is an AppImage but I couldn't figure out how to get Ant+ devices (cycle trainer and heartrate strap) working on a distro besides GC's documentation for Ubuntu. It worked for me on Mint, I just want to transition to a Plasma DE. Does this look like something you would know how to get working on Kinoite?