What would be the key deciding factors for me to decide whether to install Ubuntu Mate or Linux Mint Mate?
closed as primarily opinion-based by Jeff Schaller♦, garethTheRed, Stephen Rauch, G-Man, LinuxSecurityFreak Nov 19 '17 at 13:03
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.
Linux Mint is based upon Ubuntu, but does not adhere to the Canonical standards, thus cannot be called Ubuntu, and Linux Mint Mate is a flavour of Linux Mint.
Now, what is the difference to you?
Well, if you ask your questions here, there really is none, but if you intend to ask questions on Ask Ubuntu about Linux Mint Mate, they will close your questions and tell you to ask your questions here if you use any variant of Linux Mint (including the Mate version)...
As to the key deciding factors, well those are totally subjective, so off-topic here and totally up to you.
The only packages that are in the Mint (18.2 serena dist) main pool are:
blueberry, caja-folder-color-switcher, cinnamon-core, cinnamon-desktop-environment, cinnamon-themes, folder-color-switcher, grub2-theme-mint, ia32-libs, iso-flag-png, libreoffice-style-mint, linux-kernel-generic, linuxmint-keyring, mate-desktop-environment, mate-desktop-environment-core, mint-artwork-cinnamon, mint-artwork-common, mint-artwork-gnome, mint-artwork-kde, mint-artwork-mate, mint-artwork-xfce, mint-backgrounds-maya, mint-backgrounds-nadia, mint-backgrounds-olivia, mint-backgrounds-petra, mint-backgrounds-qiana, mint-backgrounds-rafaela, mint-backgrounds-rebecca, mint-backgrounds-retro, mint-backgrounds-rosa, mint-backgrounds-sarah, mint-backgrounds-serena, mint-backgrounds-xfce, mint-common, mint-dev-tools, mint-info-cinnamon, mint-info-kde, mint-info-mate, mint-info-xfce, mint-mdm-themes, mint-mdm-themes-gdm, mint-mdm-themes-html, mint-meta-cinnamon, mint-meta-codecs, mint-meta-codecs-core, mint-meta-codecs-kde, mint-meta-core, mint-meta-kde, mint-meta-mate, mint-meta-xfce, mint-mirrors, mint-themes, mint-themes-gtk3, mint-translations, mint-upgrade-info, mint-user-guide-cinnamon, mint-user-guide-kde, mint-user-guide-mate, mint-user-guide-xfce, mint-x-icons, mint-y-icons, mint-y-theme, mintbackup, mintdesktop, mintdrivers, mintinstall, mintinstall-icons, mintlocale, mintmenu, mintnanny, mintstick, mintsystem, mintupdate, mintupload, mintwelcome, nemo-folder-color-switcher, nvidia-prime-applet, rhythmbox-plugin-tray-icon, syslinux-themes-linuxmint-cinnamon, syslinux-themes-linuxmint-kde, syslinux-themes-linuxmint-mate, syslinux-themes-linuxmint-xfce, ubiquity-slideshow-mint, ubiquity-slideshow-mint-kde, ubuntu-system-adjustments
Everything else is straight Ubuntu, from the Ubuntu mirror set.
Basically it comes down to default config, artwork, etc.
A Mint 18.x desktop looks better on first boot than an Ubuntu 16.04 desktop, even with MATE installed on the Ubuntu system.