I'd go with Mint.<BORING STUFF YOU CAN SKIP>
The main difference between Ubuntu and Kubuntu is that the former uses Gnome and the latter uses KDE.
These are the two most popular of something called a "desktop environment." They are responsible for creating the default set of buttons, checkboxes, radio buttons, sliders, and other controls used by programs to create user interfaces. They also provide the "desktop" where you can put icons and such, the menubar (which is similar to the Mac OS menu in Gnome and the Windows taskbar/start menu in KDE)
Each of these desktop environments comes with its own set of default applications. For example in Gnome the default file manager is called Nautilus whereas in KDE is Konqueror (analogous to Windows Explorer or Mac OS' Finder), the default email program is Evolution(Gnome) or Kmail(KDE).
The reason why there are these two (actually there are more than two, but these are the big ones) competing desktop environments is licensing.
THousands of years agon in the 1990s there was an upraor in the Linuc community over the fact that KDE was becoming the defacto standard desktop environment for Linux. This was because KDE was written using the proprietary Qt library and was, therefore, not open-source. The Gnome project was started with the aim to supplant KDE by creating a desktop environment using the open source GTK+ library.
Qt has since moved into the realm of open source but the two camps still persist.
It's largely a matter of deciding which environment you like best and using that one, but both can be installed and selected by the user at logon time, plus most programs written for one environment will work just fine under the other if the proper libraries are installed.</BORING STUFF YOU CAN SKIP>
There are others you can choose from too, like Xfce which is used in Xubuntu and is designed to be ultra-lightweight for older computers.
Or you can go totally hard core and not install any environment and just do everything from a terminal prompt, which would automatically make you a level 37 geek.
Edited by Amazing Andrew, 17 August 2009 - 02:43 AM.