Introduction

People continue to ask "which Desktop Environment should I choose?" and the answer still is "It depends". It depends what you do with your DE and which apps you commonly use.
Kde4 is very fast and responsive (thanks Qt4), it has some new concepts behind and a new framework, but it still not complete and it is still a beta.
Gnome is a consolidated DE which adds some new features every 6 months. It also has interesting things to show but it needs a lot of time to make them mainstream. Somebody says it is in decadence.

Philosofical differencies

Kde offers to the user every settings a normal system could provide.
Gnome offers to the user only the settings a normal user could need.

There is not a correct approch, but a correct balance.

Every day usage differencies

Most of the useful features are avalaible for both desktop. You can surf smb://, ssh:// shares, send an email, organize your photos with both DEs.
Some applications are more intuitive or more featured for one DE, so you can to adapt to the app your DE provide or choose the other one equivalent.
Every DE provide an Office, Web Browser, Email client.. alternative but if you need a full-featured app, probably You'll continue to use some DE agnostic apps like Openoffice.org, Firefox, Thunderbird,..

Every day differencies