Techsoup have a massive introduction to and about Ubuntu as well as comparing it to Windows and OS X.
Read it all here
A free, stable, and secure operating system that you can try out risk-free? Open-source operating system Ubuntu comes at a tempting price and offers many of the advantages of popular commercial operating systems such as Windows and OS X. But do its potential drawbacks manual installation and possible compatibility issues outweigh its many benefits? We'll show you how Ubuntu stacks up to other operating systems and help you decide whether this increasingly popular solution is right for your nonprofit.