Well, they're both pretty good.

Xfce starts off closer to the way I like things and requires less messing around with the config to get it all the way there. It changes less frequently, which I appreciate. It uses slightly less memory, seems to go wrong less often, and has fewer things I have no interest in like 'kwallet' or whatever. And I appreciate what seems to me its more approachably modular nature (which I've used only for a couple of minor things.)

for me, simplicity.