As a follow-up to some of the posters, which window manager you use is indeed a matter of personal preference, and there have been many "flame wars" on boards over the years with followers fiercely advocating for each. And to confuse matters worse there are several versions of "Gnome" each with their own idiosyncrasies and selling points.
Most of these arguments stem from the early days of Linux. At that time many wanted to keep open source "pure" and "untainted'" releasing only software under the Linux "GPL". At that time Trolltech the developers of "QT" (the C+ environment) on which KDE was based upon was "closed sourced" and had a restrictive license even though apps built against were open sourced. This meant that developers were free to develop provided they purchase "QT". Gnome on the other hand was and is entirely written in "C" and was compiled against the open sourced GNU libraries and compilers.
Purist distros like Debian for a long time refused to even offer KDE. Mandrake/Mandriva for example, was originally created as a fork of Red-Hat because RH refused to include KDE in it's repositories. Suse was another that sprung up in order to offer a KDE environment. Several years ago Trolltech changed their licensing which in turn allowed KDE to be included in a number of additional Distros. To the point where they're both included in just about everyone's repositories.
That said, over the years the developers of Gnome couldn't get along, were working independently, kept missing release deadlines and eventually splintered into many different desktop projects. So that now there are many flavors of Gnome each a bit different from one another but built upon the same libraries and toolkits. KDE on the other hand has remained more focused, because of this KDE for a long time presented the user with a much more unified tightly knitted suite of applications.
There was a time a few years ago when every Gnome app looked different from one another, which while being lighter, less resource intensive than their KDE counterparts presented a disoriented, unorganized appearance to the user. Realizing this, a few years ago all of the major players in desktop development got together to form the Freedesktop organization with the goal of creating interoperability and more uniformity between them all. Now with that history lesson behind us, let's jump to the present.
KDE generally takes up more resources (memory, cpu-power, disk-space). Gnome is generally lighter weight (uses less resources). KDE is more unified and is usually easier to get a handle on for someone coming from the Windows or Mac world of computing. If you're starting from scratch building a Gnome system requires more homework, because there usually are several Gnome apps to do similar jobs.
Like Windows or a Mac, KDE does more things "automatically", where as in Gnome you have to play around directly with scripts or GUI's for scripts to configure to your liking. Finally, since KDE is built against QT which has a commercial entity behind it, it's constantly evolving, and as improvements and enhancements are made, they get used by the KDE developers to improve KDE. Many who once trashed QT and KDE now say that the current QT is one of the best development environments ever released.
So there you have it. It's all a matter of preference. I personally have been running KDE since I left Red-Hat to follow Mandrake when they forked in the late '90's. But I use Xfce (a lightweight alternative suite) as my back-up in case I have a problem with my KDE installation. I like Xfce because it's light but runs all KDE apps flawlessly. It's handy to have a back-up desktop to use in case a new release is buggy or fails to compile correctly.
I hope this helps.