I installed Ubuntu 8.04 and added Kubuntu from the repositories. When I log out
I can change the session at the bottom from either Gnome or KDE. Otherwise you
will get the last run desktop manager. I also have KDE 4, XFCE, Fluxbox and
Sugar installed. I don't use the others all of the time, but I like to use them
enough to be of help to others.

Gnome or KDE is chiefly about personal preference. They are equally powerful,
but KDE is more configurable and flexible. It has more tools out of the box as
well. Linux Torvalds, the founder of Linux, prefers KDE, but that does not mean
that you should. It is what works for him. I suggest that you give them both a
spin and see for yourself. I cannot decide and I have been using them for over 7
years. I flip flop and use each for variety.

I like KDE 4 and see much potential in it, but it is still not ready for prime
time IMO. It is still a bit buggy and it is not as configurable as KDE 3. It is
faster, though and is much improved over when it first came out. Then, it was
large and clunky and you could not configure anything. It is getting better all
of the time. They hope to release a Windows version this year. how is that for
ambition. Soon Windows and Linux can share the same look and feel!

XFCE and Fluxbox are good for computers where resources are limited. They work
fast and well, but are not feature rich. Sugar is the OS on the OLPC computer
and is a toy, although I know of someone who programs on it. You can also
install Enlightenment which is something to try once you get some experience. It
is for people who want to take more ownership over the interface. There are a
host of other managers as well.

You can run more than one at once and some will even operate inside others.
Ctrl-Alt-F7 or 8 will switch between two different sessions. For example, in
some distros you can run as user in one and as root in another and switch
between them with a keypress. Don't try that until you are more experienced,
though. Ctrl-Alt-backspace kills any session and brings you back to the login
screen.