1. Linux is free.
This is a huge factor. Linux is free to install. When an update comes along, it's free too. The majority of the software is free. Not having to pay for the OS and updates to the OS can be a huge long term cost savings for your buisness. When you buy a computer with Windows on it, you are paying the "Windows tax." Part of the money you paid for that computer goes to Microsoft. If you decide you don't want Windows on it and you call up the manufactuer and you tell them that, in many cases they will refund you a portion of the cost of the computer.
2. Linux is Reliable
There are computers and servers out there running Linux that have uptimes of years. Why? Linux takes the user into consideration and tries to make restarts few and far between.
3. Linux is Secure
Every year, millions of dollars worth of important data is lost in virus and malware attacks. Don't let your buisness be a target. The number of viruses written for Linux operating systems is a fraction of that of Windows and Mac OSs. Why? Because Linux is a lot harder target then Windows and Mac. Linux patches security vulnerabilities as soon as they are discovered, with many people poring over the code, looking for code that might cause problems. All software in the Ubuntu Software Center is virus-scanned. In Linux, no user has too much power. In order to perform any administrative action, you need to re-enter your password. This stop many malware attacks in their tracks.
4. Linux is user-friendly
Linux has may features that set it aside from the other OSs that make the everyday user experience enjoyable. Things like the Software Center. Having all sodtware in a central repository in which other people can review and provide feedback on the app helps the users decide whether or not they want to download the app and the developers to fix any problems that users might be having.
5. Linux is Customizable
Your Linux is your Linux. Feel free to change things you don't like and add things you do, and share those changes with the community. Easy to use graphical tools let you tweak your Linux experience to just the way you like it.
After all of this, you are probably thinking to yourself "Man, this Linux thing sounds cool, but what makes Ubuntu so special?" It's simple. Ubuntu was designed from the ground up with the user in mind. Super user friendly and with a community of users willing to help just a few mouse clicks away, Ubuntu is Linux for Human Beings.
That's why you should use Ubuntu. Don't believe me? Ask anyone on this site, they'll be glad to tell you thier experiences. Still wondering is a different Linux distro might be better? Go ahead, try it out, there's nothing stopping you. Linux is all about freedom.
Still want more info? Check out http://whylinuxisbetter.net . There you'll find all the reasons I've listed and more, with proof to back them up.