KDE and Gnome are different ways of doing the same thing - managing your
desktop; You will find disciples of both.

Basically - and I know I'm over generalising here - Gnome is simpler
while KDE is more configurable. However, you don't need to worry which
version of Ubuntu you're downloading as you can install both at the same
time then, when you logon, you can choose which to log into. My advice
is to try them both and see which you prefer.

As to applications, so long as you have the appropriate KDE/Gnome
libraries installed, you can pretty much run any KDE app under Gnome and
vice versa.