Xfce and MATE are fairly similar in a lot of ways. Back in the day Xfce had more emphasis on being lightweight than Gnome 2 did.
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.
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.