You should also consider KDE Neon, an 18.04 base but KDE/Plasma directly from the devs. If you use Snaps for your useland tools you have very stable base and a super up-to-date userland. I'm very happy with it.
Just wanted to add my support for KDE neon. I used kubuntu back in 16.04 and switched to neon instead.of the next LTS kubuntu and I haven't regretted it. Plasma feels a lot more stable and up to date than it did with kubuntu and I haven't had any problems since switching.