they are very different...
Ubuntu is Debian based, totally precompiled,
Sabayon is a optimized compiled ( Entropy) or compilation distro ( using Portage the packages are compiled in the comp, from source, from ebuilds) also is a modified Gentoo Linux.
Ubuntu is more easy to use, but is also much more limited.... Sabayon is not dificcult, but much more customizable and may be optimized for your machine.
Ubuntu is more easy to use, but less profissional, or customer modified, the proprietary drivers,as NVIDIA for compiz, is not installed by default in Ubuntu, but are installed in Sabayon.
The package manager in Ubuntu is apt, using the commands apt-get,ou aptitude or the GUI synaptic, therefore Sabayon uses Entropy installer, command equo, or GUI spritz, or Portage, command emerge (is the compiling installer from the North American Linux Gentoo.
for using Ubuntu,you may see inthe net documentation of Ubuntu itself or Debian, or Mint (an sophisticated modified Ubuntu),but for Sabayon, you may see Sabayon or Gentoo documentation;
Basically, Ubuntu is Debian like, many other distros also... Sabayon is a n Gentoo, and Mandriva, fedora and OpenSUSE are modified Red-Hat Linuxes... all are different, including commands, repositories and packages.....
Ubuntu is updatable, but sabayon is updatable, bu also Bleeding Edge, viz if you want, you may install even trunk or unstable packages, it is much more updatable than Ubuntu
the number od installed packages, by default, sabayon have more packages, is more complete. Mandriva also....
i tested more than 40 Linux Distros. I prefer now : Sabayon, Gentoo and Mandriva free....
OpenSUSE, Fedora, Mint,Elive etc... are good linuxes, but i never seen Distro like Sabayon! is my Favorite Linux.
In my oppinion, despite is an personal opinion, Sabayon is much better.
ALBERTO FEDERMAN NETOalbfnetoalbfneto@fcfrp.usp.brFavorite Distros: Sabayon, Gentoo, Mandriva and OpenSUSE