In terms of being able to put your UI elements wherever you want, being simple, configurable and non-opinionated, it's all very similar. I would assume (could be wrong) that Xfce is under more active development than Mate.
I made the switch when Gnome 2 was killed overnight, and Mate didn't yet exist. My Xfce desktop looks a lot like my Gnome 2 desktops used to. I've never felt the need to switch to anything else or 'back' to Mate.
One of the things I love about XFCE is its modularity. It's literally just a collection of programs that work independently, so while I use DWM, if I need a panel, I just type "panel" into dmenu, and XFCE panel runs right on top of it with no problems, aligning perfectly over the DWM top bar.
If you want to try a more complete DE, I'd recommend COSMIC. It's fresh and fast and very customizable.
I first used it on an eeepc because something light was the order of the day. But then Gnome 3 happened and I made the switch on my full-strength machines too.
It works and it works well. It's theme-able. It's not opinionated about how I should use it so I can put bars wherever I want, launchers, menus, systrays wherever I like, and I can do it all with a few clicks and dragging and dropping stuff.
Generally a great DE and one that won't screw you over on update, which is something I've come to value.
Post-2010ish Gnome and kde are like some sort of sick joke. The fact that there are people who actually contribute their precious free time to these, feels to me profoundly sad.
fvwm is better
Only one thing I wish I could set, allow windows to cover the 'bar'. Yes, I can make the bar auto-hide, but I cannot have a portion of it covered by a window.
The same is true for GNOME, but KDE it is allowed. I expect this is a gtk thing.
XFCE is when my backup when I break xmonad.