The main difference is that ubuntu server does not provide GUI interface Gnome or KDE for Kubuntu. Ubuntu server runs in command-line mode only without GUI.
As stated on CentOS vs. Ubuntu, Ubuntu's key feature, the package manager, is the main advantage over the competitors.
As far as the kernel goes, I would advise to compare the kernel version between releases/distros.
According to http://www.markshuttleworth.com/archives/1072, Ubuntu has been gaining market share in the web server competition. My guess this is mainly caused by an improved user experience in current Ubuntu versions (more lazy point-click admins ;)).
It all comes down what your user preferences are, Linux (and Debian in particular) is, well, like any Linux; stable.
The main difference is that ubuntu server does not provide GUI interface Gnome or KDE for Kubuntu. Ubuntu server runs in command-line mode only without GUI.
As stated on CentOS vs. Ubuntu, Ubuntu's key feature, the package manager, is the main advantage over the competitors. As far as the kernel goes, I would advise to compare the kernel version between releases/distros.
According to http://www.markshuttleworth.com/archives/1072, Ubuntu has been gaining market share in the web server competition. My guess this is mainly caused by an improved user experience in current Ubuntu versions (more lazy point-click admins ;)). It all comes down what your user preferences are, Linux (and Debian in particular) is, well, like any Linux; stable.
Ubuntu for desktop has your desktop and multimedia packages. It's just easier for the end user who wants to use it for desktop.
Ubuntu server has bare minimum packages without the desktop applications (GNOME/KDE/XFCE). You get packages like apache, bind and such.
You can install a desktop and make it as a server by adding/removing appropriate packages and vice versa.
The server may have a kernel optimized for server use and bundle drivers for server hardware. Likewise for desktop distribution as well.