Which one do you use and why?
Personally I use XFCE because the computer I'm running on (digitalfortress @ 433MHz, 6gb partition for Slack including swap, 8mb or less video card, 384mb RAM) is extremely old and would asplode on KDE's pretty-much-Windows GUI. XFCE also looks neat and I found it to be easier to tweak (without editing the source code) than KDE.