"Some apps will only run under one or the other?"

I may not have been clear... The point i was making is that e.g. k3b
(the burning programme) is a KDE app, when installed on gnome it will
need to install quite a lot of dependencies - compared with the number
that it will need to install if installed on KDE [but it is installed
on KDE by default].
Hence installing Kubuntu saves me downloading a lot of the extra
dependencies for running certain apps on gnome.

What i wish to know is the stability between them.