Ubuntu used GNOME as its desktop. Kubuntu 8.10 uses KDE4.1 as its desktop. GNOME
has its own desktop display manager, GDM, as does KDE, KDM. You can only use one
display manager, either GDM or KDM. You can have both GNOME and KDE installed,
though, and one will use the other's display manager. GNOME uses Metacity as its
window manager and KDE has its own, Kwin. Both now support compositing, but you
can run Compiz if you prefer. GNOME has its own decorations and so does KDE.
However, you can use GNOME decorations in KDE, or vice versa or you can use a
third party decorator such as Emerald. I am sure that all of this is clear as
mud. Basically they are full desktops with interchangeable parts. GNOME is
written with GTK and KDE uses QT4, so when you run parts of one in the other you
need to have the runtime libraries for the other installed. This is true even if
you just use one KDE programme such as Amarok in Ubuntu.

KDE is older than GNOME, but it has been recently totally re-written using the
latest version of QT. GNOME is due for a similar re-write sometime soon. GNOME
is meant to be simpler to use with fewer configuration options, however, since
KDE 4 came out features have been added on the fly and they are roughly the same
at this moment. KDE 3.5 was much more robust than GNOME and KDE4 will be more
feature packed when it is done. GNOME uses mono, something which I personally
deplore, since it is dancing with the devil so to speak. Mono is the open source
implementation of .NET from you know who. It was produced by Novell who signed a
controversial agreement with you know who. This agreement has led to sites
devoted to boycotting Novell and openSUSE Linux from Novell has been in decline
ever since.I personally will not use any mono programmes, but this is my own
hang up. I use GNOME and in fact am using it at this moment. I just de-install
mono and any
programmes such as Tomboy, Banshee, Gnome-do, Drapes, gBrainy, Fspot and
Gtwitter or any others that rely on it. Otherwise GNOME is great! So is KDE, it
is my desktop of choice most days.

You can also install other desktops. These would be good choices if your system
is sluggish with any of the others. Install XFCE or the Xubuntu metapackage.
Lxde is being used more and more often and Fluxbox continues to be popular as an
alternative to GNOME, KDE or XFCE. Why not try them out and see what works best
for you? You can choose a different desktop at login by going to Session.