For me, the main difference between MATE and Cinnamon, is the fact, that Cinnamon is mainly used by Linux Mint users, while Mint is much widely adopted. A lot Linux distros using it by default or have MATE as installation option, also BSD systems using it too, and OpenIndiana uses MATE too. So MATE is a truly widely used Unix desktop, and I really like it about MATE
But anyway, since Ubuntu switch to GNOME, I think, many people will switch too, to KDE, LXQt, CInnamon (Mint) or MATE, or to a different OS too. But anyway, it will definitely increases the MATE userbase.
But what is came in my mind right now, Ubuntu will be more similar to Fedora, since both will use GNOME, the difference is the default theme, and one is Debian based, while the other is Red Hat based. I wonder, how will incease/decrease the two's userbase this.