Before the desktop environment is loaded. it is the same, including package
management, etc. What is different is kdm vs gdm, but when you install both
it asks you which one to use. I have gone both ways and don't honestly
prefer one over the other. If you use XFCE as well then go with gdm. Having
more than one desktop on the same system is advantageous. If there is a
problem with one then you can just load the other, fix it and log out. I
also like the change. I am GNOME at the moment, but have been in KDE for the
past couple of weeks. I like GNOME,but prefer KDE. Sometimes in GNOME I will
use Dolphin as I prefer it for file management, but occasionally I will use
Nautilus in KDE because it works differently and I want a feature.

The big differences between Ubuntu and Kubuntu are they way they are
developed. Kubuntu is developed on the Canonical base, but the project has
less interference. Most of the developers are from KDE and not Canonical. It
is the opposite with Ubuntu which is Canonical's flagship and there is now
much strain due to Canonical stepping away from GNOME. The strain has been
around for some time and the move to Unity ices it. They use to same kernel,
installer (Ubiquity), modules, headers, graphics drivers and plymouth. Then
either gdm or kdm is loaded and you get X. Then you can choose any installed
desktop environment. fromt he session manager

I have run just Kubuntu and just Ubuntu, and had them both together and
cannot tell the difference. So if you have the time, install KDE in your
Ubuntu or vice versa, then import your files and settings. Then you can
delete the partition and free up the space. No matter how big your drives
you can always use the space and you simplify the boot process and gain the
advantage of having an alternate desktop without rebooting.