KDE without Akonadi which is used for Kmail and Kontact, uses about 400 Mb on a fresh start up. Yes, maybe slightly more than Xfce4, but this depends on the programs your run by default. The stability is in my experience not so much a KDE problem as well the way the developers of the respectively distros implement the DE.
I use Xfce4 as my fallback, and loving it. KDE is my main DE though.
The more programs you run at the same time the more CPU and RAM is required. This is the same for both DEs.
The Antergos implementation of Xfce4 and Plasma is stylish and very stable. No major problems with any of the two.