Well dude these are the desktop systems of Linux like Windows have only one system by default i.e. Windows GUI. There are alot more other desktop systems available for linux but KDE and GNOME are considered as standard. If you are confused about choosing which is better then don't get confused it depends totally on your needs...
Your need for speed:
One of the big differences between the GUI is the relative workload they put on your system. The more a GUI has goodies, features and eye candies, the more it will need power to run properly and the slower it will be.
KDE and Gnome are two of the most developed, and heavy, GUI available for Linux. Thus, if speed is of prime importance for you (for example, you want the maximum of power to run a task or you have a slow PC), you may be better to use a lightweight GUI, like IceWM, Fluxbox or Afterstep. In other word, the lighter your GUI is, the faster your overall system will be...
Which applications are you using?
The fact is that some applications just run better in one GUI than others.
KDE and Gnome are based on a different set of programming libraries and tools (named GTK for Gnome and Qt for KDE). Thus, it's easy to understand that applications designed and based on the same GTK libraries than Gnome will work better under Gnome than under KDE. And the reverse will be true for applications based on Qt.
So, it may be better to use the GUI that works best with your applications. If don't know the libraries used by your applications, just try them with different GUI and see if it makes any significant difference. For some applications it will, for others it won't. Thus, your favourite applications may help you determine which GUI you should use.
Conviviality and ease of use:
This point is more a personal choice, some peoples may find that KDE is convivial and easy to use, but others will be overwhelm with the hundreds of features and options... The choice is up to you, each GUI is, more or less, easy to use, you just have to make the best choice for you...
However, if you're familiar with Windows, you may feel more at ease with Gnome or KDE, which are similar, than with a less classic GUI (like Enlightenment for example).
The default GUI of your distribution:
All distribution give preferential support and integration to one or several GUI. If you have Mandrake, both KDE and Gnome are equally supported. But Red Hat never hides their preference for Gnome, and SuSe for KDE...
All other things being equal, it may be a good idea to use your distribution's favourite GUI. It will be less hassle since it's the default installation and configuration. Integration and the "polish" will also be better, default settings being usually just fine. It's also easier to get support, since most peoples using this distribution will use the default GUI.
Your need for eye candies:
All the GUI have a different look, some are gorgeous with lot of effects, font anti-aliasing, nice icons, and all kind of goodies, others look pale and simple in comparison. The choice between these two extremes is only a matter of personal preferences. Some will prefer a GUI with lot of eye candies, others just want a clean desktop on a simple background. None is better than the other, it's up to you to make your choice...
Final words:
With these points in mind, you should now be more able to choose a GUI. However, remember that choosing a GUI doesn't mean that you have to stick with it for the rest of your life! Nothing prevents you to install many GUI on your system. Then you choose the one you want when you log in. You may choose the GUI that's the most appropriate for the task...