D'après ce commit de Joey Hess sur Tasksel (l'outil d'installation des paquets), Debian utiliserait donc XFCE par défaut :
Le message de commit :
switch default desktop task to xfce
This ensures that the desktop will fit on CD#1, which gnome currently does not.
There may be other reasons to prefer xfce as the default as well, but that
is a complex and subjective topic. Unfortunatly, Debian does not have a
well-defined procedure for making such choices, though it (…)