Both are desktop environments. KDE generally has more features and options, Gnome aims to reach simplicity but foregoes options/features. You can have both installed and try them out.
To the original poster: What do you want to achieve? Just run some linux software? Then you don't really need linux and might use recompiled versions directly on OS X (with X11 installed). Might want to take a look at our UNIX & X11 forum here on macosx.com.