Ok, i'm going to throw something into the mix,
The differences between Ubuntu and CentOS are;
Ubuntu is based on the Debian distrobution and by default uses the .DEB packages, for updating and installing new software the package manager would be apt which has a graphical tool called synaptic, it is made to be an easier distribution for newcomers, it's not the most bleeding edge distribution out there although the main releases do try to stay as close to it as possible.
There is also a LTS (Long Term Support) branch which will stay with older and more stable packages.
The community i find for the most part to be extremly welcoming and helpfull, most of the advice i have found to be sound and right.
CentOS is a free version based on the RedHat enterprise suite, because it is based on RedHat it is an .RPM based distribution and uses that for packages rather than .DEB, it uses yum rather than apt to install/update its packages.
Because it is designed for enterprise desktops/servers the programs/kernel et al have to be stable so it relies on older packages (note this will not stop you installing newer apps by hand ie, rpm -Uvh <installer>.rpm)
both can still install from source packages and ubuntu can use alien which will allow (or convert to DEB, i forget) you to install RPM packages on it.
Both have their merits, i would choose Ubuntu as it's currently my weapon of choice for my home systems
Stick to the LTS versions if you want the stable package base!!
New version is out in 6 days!! and the LTS will be coming out in 2010.
"You'll never see all the places, or read all the books, but fortunately,
they're not all recommended."