None of the two big desktop envirnments shoul be used in a Live system. Both Gnome and KDE are big, sluggish and bloated. This is not much an issue when you use them in a traditional OS, but if you are running from a Live CD, lightweight and speed are mandatory. Let's face it: KDE4 is slow even when it runs from RAM. Bad thing. Gnome would be even worse, as its dependencies are very invasive. Damn, Gnome3 demands you to install systemd!!!
XFCE is faster than the former, but it uses more resources that people uses to believe.
For me, the better options are LXDE, some other standalone window manager (Openbox, Fluxbox) or even a tiling window manager (wmii). They don't demand as many dependencies, are light, take less room...
The final decision depends of the target user of Porteus. Do you target Geeks? No? Then forget about wmii. KDE and Gnome have plenty of wizards to keep new guys happy, but also to annoy me

and slow Porteus down. They are supposed to be more funtional, well... I don't really think so. I can do with LXDE what Gnome users can do with a Gnome wizard, but LXDE starts in no time and Gnome lasts an eternity.