I hope this discussion should continue, rather than conclude, as more views means more purity in idea.
As far as security is concerned, it is the Debian OS which determines policies more than the DE. Also, I would like to categorize them as under:
KDE : Heavy, high-end graphics, consumes resources (
GNOME : Moderate, cool graphics, consumes moderate resources,
Xfce : Light, not-so-good graphics, customizable, consumes less resources
MATE : Moderate, cool graphics, consumes moderate resources, BASED ON GNOME2
LXDE : Extremely light, XP type looks, minimal resource consumption
For everyday use, I prefer to GNOME (gnome3), personal opinion, while many people hate the changes, but it's pretty good.
You can prefer to MATE if you were a gnome user and were used to it.
You should use XFCE or LXDE if you don't care about the looks but the performance.
Also, for a lazy weekend, if you have DVD1 of the iso image OR good internet, then you can install any desktop environment and have a try yourself rather than having other's opinions. That would let you know what suits you.
Disk space is not a big issue these days. :)