https://launchpad.net/~kubuntu-ppa/+archive/ubuntu/backports
https://launchpad.net/~kubuntu-ppa/+archive/ubuntu/ppa
The first PPA is called "Kubuntu Backports" and the second PPA is called "Kubuntu Updates".
Which one of those two PPAs should I add to Kubuntu/Ubuntu in order to get the best and the most up to date KDE Plasma? Or should I add both PPAs?
I would suggest installing only the
kubuntu-backports
ppa when it becomes available for 18.04.from https://launchpad.net/~kubuntu-ppa/+archive/ubuntu/backports
Also, you could monitor https://www.kubuntuforums.net/forumdisplay.php/30-Kubuntu-Announcements where Kubuntu devs will announce the kubuntu-backports releases.
You could also monitor The kubuntu-devel Archives and, in particular https://lists.ubuntu.com/archives/kubuntu-devel/2018-May/011659.html as to what exactly will be provided via the kubuntu-backports ppa in 18.04.