Like
vansmith said, it's more a question of "different" than "better" or "worse".
Also, it depends what you plan on doing with Ubuntu. If you have an old PC laying around somewhere definitely install Ubuntu on that; where it's got the same kind of strong UNIX base as OSX (Ubuntu is based off of Linux, & OSX off of BSD, but both Linux and BSD are based on UNIX), it will be as stable as OSX would be on a normal PC.
I.e. an Ubuntu PC is the closest you'll ever get to a legal Hackintosh, you can even change the theme to make it look like OSX.
However, if you install Ubuntu on your Mac it'll be no more stable than if installed on a PC (both are just computers to Linux).
That said, considering it as a secondary OS for your Mac, there are a few pros:
• As stated above it's more stable than Windows, even if not quite as good as OSX.
• It's free, versus Windows's hefty price-tag.
• It's
virus & malware free, like OSX.
• There's at least as much freeware for Ubuntu as for Windows, it's easier to find (most being in a repository that can be accessed form the OS's application manager), and where it's all developed by one user/developer community it has a similar level of uniformity of standards & neat integration with the OS as Mac apps for OSX.
• It'll have an easier time than Windows accessing your personal files from your Macintosh partition should something go horribly wrong with your OSX install (hardly ever happens, but anything's possible), due to the more-similar architecture.
• It's probably the most customisable OS there is, with thousands of themes available (including ones to make it identical to OSX, as said above, or even Windows 7), and practically all of the guts of the OS is rewritable too, if you dare!
That's all I can think of for now...
Good luck and happy computing