I am willing to use KDE Plasma 5 Desktop environment.
Should I use Kubuntu or Ubuntu 16.04?
If I would use Kubuntu, which advantages would it offer?
Kubuntu is what you want.
Kubuntu is just Ubuntu, but without Unity and with KDE instead; that's why it's called Kubuntu. (KDE Ubuntu)
Ubuntu has nothing to do with KDE. You can install it, but you'll have to switch from Unity, and you'll run into theme issues.
I'm not sure where the confusion is honestly :p. I'm pretty sure Kubuntu says it uses KDE.