Hello Alair,
From what I can tell, Xfce is lighter on resources than MATE. My best guess for resource usage of UIs or DEs --from lightest to heaviest-- would go something like this:
Command line only, Windows Managers (e.g. Openbox, Fluxbox, etc.), an LXDE-style DE (e.g. Peppermint), Xfce (e.g. Xubuntu), MATE (e.g. Ubuntu MATE), GNOME (e.g. Ubuntu GNOME), Ubuntu, KDE (e.g. Kubuntu).
I can personally attest that Xubuntu is as solid as a rock! MATE is very popular too, and I’ve never heard anyone complain about it --ever. Personally, I like Xfce better than MATE. If you like the menu of MATE, then I would suggest trying Peppermint --because, in my opinion, the menu is even easier to use than MATE’s, and you also get the added benefit of having Peppermint’s support forum too.
I don’t know where you heard that MATE was on its way out. Although MATE is based on an older style of GNOME, MATE seems to be increasing in popularity over time --not decreasing. I guess going retro is hip these days! But, lets not forget, Xfce is retro too --twenty years old! Xfce just isn’t getting the press it deserves. 
For lightness and practicality, I, personally, would either go for Peppermint or Xubuntu. But, in fairness to Ubuntu MATE, it’s supposed to be quite good on resources too, and a lot of people now think it’s the best flavor of Ubuntu around.
Alair, why not try all three of these distros for yourself --using either a flash drive or some inexpensive DVD-Rs. Peppermint, Xubuntu, and Ubuntu MATE are all winners really! I don’t think you can go wrong with any one of them. 
Alair, I don’t know why there are so many terminal options with Xfce. I just pick the one that says terminal, or terminal emulator. I pick the one that looks like a normal terminal, and then I use it and forget about the others. But your question is still a good one. But don’t let it bug you. There must be some reason for having two terminals. These developers really do know what they’re doing. Whatever the reason, it’s not accidental. 
perknh