Okay so I have the last version of ubuntu running on my computer along with vista which I hate its so un-reliable,Im thinking of installing Cent OS and running my own private server on it,which OS do you find better Ubuntu or Cent OS?
Out of the choice between ubuntu and centos I would probably choose ubuntu. Reasons being mostly that ubuntu has a great community and is quite user-friendly. Centos is an enterprise distribution and wouldn't exactly be the most-friendly for a home user. It really depends on what server you'll be running.
The best distribution in my opinion is Mint Linux: http://www.linuxmint.com/
It is based on ubuntu, but is really easy to use. It includes "mint tools" and a software manager and package manager so you can easily update, install, uninstall software. It has one of the most simple control panels of any distro I used.
Best of all, it's only 1 of the 2 distros that ran without a hitch on my laptop, including the detection and proper use of my wireless card! (The other distro was puppylinux, but really a liveCD distro)
Well, there are many things I can say in favor of Ubuntu and there will be many others on April 2010. The most valuable thing about Ubuntu is the desktop orientation, Ubuntu users are in a large majority desktop users. Also the artwork coherence (from boot loader to default desktop) is remarkable. Anyway it's good to know that if something goes wrong, there is a really big community support that will help you to solve your problem. If you need enterprise class support, then Canonical can offer you some plans.
CentOS, in the other hand, is good at some things, I'll prefer it as a server, but for workstation's role, I think Ubuntu is one step forward. In addition, the community using CentOS as a workstation is not as large as Ubuntu's, so it's possible that an answer(support) will arrive but not as fast as with Ubuntu.
And finally, Ubuntu offers PPA and backports alternatives, I think there's nothing similar on CentOS.
Exactly as albuferque said: Ubuntu for a Desktop, and CentOS as a server