I have not found current Sabayon to be unstable, I had problems a couple of years ago but it seems fine now. Also Sabayon does not use all unstable packages for example it uses Python 2.7.2 (3.2.2 unstable) and bash 4.1_p9 (4.2_p20 unstable).
In most cases packages marked testing/unstable have already been declared stable by the software developers. The extra testing that gets done before marking the package as stable is done by distro developers.
In the case of Sabayon, as has been pointed out, there are 2 sets of developers testing the software which will tend to make it more stable. Debian doesn't have that.
When comparing distros, using the term 'better' is not very useful, it is just an opinion. I think focusing on strengths, weaknesses and differences is more useful.
Strengths: many packages to choose from, apt is an excellent software management tool.
Weaknesses: can be challenging to install, some packages are very old and have not been updated for years
Strengths: many packages to choose from, equo is an excellent software management tool, a variety of DEs fully configured and ready to use, multimedia support built in, easy installer that only requires a few minutes to answer some simple questions.
Weaknesses: same as all rolling distros, from time to time an upgrade will break your system.
Just because something is 'stable' doesn't mean it is secure. The latest Debian stable is called squeezy. It was released last Feb (typical time between stable releases is 2 years). If you install it now, you need to apply all security patches that have been released in the last year. Not a trivial task.