Ubuntu – Lubuntu as an official derivative of Ubuntu

There is an xubuntu live (desktop) cd available here :

http://xubuntu.org/getxubuntu/

xfce is a desktop environment , see http://www.xfce.org/ and http://xwinman.org/

So when you install xfce you get xfce.

xubuntu, however, is an additional set of packages and customizations to make the xfce experience more rich. xubuntu-desktop is a meta package and you can see the entire list of packages here

http://www.ubuntu.com/about/about-ubuntu/derivatives

As you can see the list includes xfce + a number of additional packages.

See also https://help.ubuntu.com/community/Installation/LowMemorySystems#XFCE

Use which ever you prefer, from your post I would suggest you stay with Ubuntu (unity).

Just install the one that you want. For example to install LXDE, just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below.

sudo apt-get install lxde

for XFCE

sudo apt-get install xubuntu-desktop

for Gnome

sudo add-apt-repository ppa:gnome3-team/gnome3

sudo apt-get update

sudo apt-get install gnome-shell

Once you install the one you want, log out, choose the one you want, and log back in.