If I understand you correctly, you replaced Ubuntu (with only Gnome/Mate) by another distro Kubuntu with only (KDE). Is that correct? I suggested Debian if you want to change from one Desktop to another. You can have several desktops in Debian by just adding the relevant packages. For example if you have installed the Debian default desktop (Gnome) and want to add KDE you can install the kde-full package. Here's the description:
Description: complete KDE Software Compilation for end users
KDE is the powerful, integrated, and easy-to-use Free Software desktop platform and suite of applications.
This metapackage includes all the official modules released with KDE Sotware Compilation that are not
specific to development and as well other KDE applications that are useful for a desktop user. This
includes multimedia, networking, graphics, education, games, system administration tools, and other
artwork and utilities.
If Kubuntu only gives you one Desktop then you might not get a Desktop menu in your login screen. On the login screen you should see 2 icons on the bottom left corner of the login window. The one of the left is a circle with gears on it. That's where you'll find the option of another desktop if you have one installed. If (K)ubuntu works like Debian you could add the Gnome desktop but simply giving the command
sudo aptitude install gnome
Installing the gnome desktop and all its dependencies will involve about 350 packages.
Note that all my info is based on Debian. I have no knowledge/experience with Ubuntu sooooooooo. YMMV.