The difference between Xubuntu and Kubuntu is the windowing systems they use. Regular Ubuntu uses what is known as Gnome, Xubuntu uses the light and easy Xfce windowing system and Kubuntu uses the also popular KDE.
Xubuntu is best for running on lower end computers since XFCE doesn't take much toll on the RAM or CPU to run. When picking a version always get the latest one because unlike the Windows family of operating systems the newer they are the better they will run.
If you want to boot from a USB drive then I'd say totally forget about Ubuntu and install NimbleX or Dyne:bolic instead. NimbleX is a fully customisable system and you can make your own on their website. It uses the KDE windowing system and is based on what is known as Slackware, a version of Linux optimised for running off of USB drives. I've been making use of NimbleX for a long time and haven't faced any problems with it so far. NimbleX can even fit right down onto just a 128mb USB drive with it's sub100 version. You just pop in the CD, it will boot the OS from the CD, then in the Home folder you will find the USB installer. If your computer doesn't boot from USB then you are still in luck because when you place the CD in at the beginning all you need to do is press F2, then type probeusb from=/dev/sda1/nimblex. Moist.
19/01/2008
pinked - Kiama, NSW