Ubuntu MATE will surely use less resources than just choosing MATE from DE since Ubuntu-Desktop installs many packages with unity and Ubuntu-MATE has very few dependencies.
Thus if you just install MATE DE in default Ubuntu, the packages which were installed with unity will be there and some of them maybe running in background, thereby using more resources than Ubuntu MATE fresh install.
But if you want to convert Ubuntu into Ubuntu MATE without wiping you may follow steps below, it will have the same effect as that of installing a fresh instance of Ubuntu MATE.
sudo apt-add-repository ppa:ubuntu-mate-dev/ppa
sudo apt-add-repository ppa:ubuntu-mate-dev/trusty-mate
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install ubuntu-mate-core ubuntu-mate-desktop
sudo apt-get install mate-desktop-environment-extras
Also install gdm by this command :
sudo apt-get install gdm
Then you may remove ubuntu-desktop by following command :
sudo apt-get remove ubuntu-desktop --purge
sudo apt-get remove compizconfig-settings-manager
sudo apt-get remove compiz-fusion-plugins-extra
sudo apt-get remove compiz-plugins-extra
sudo apt-get purge compiz*
sudo apt-get remove unity
sudo apt-get remove unity-2d
rm -rf .config .gnome .gnome2 .compiz .cache
An alternative to Ubuntu MATE is lubuntu which is even light-weight than Ubuntu MATE, choice is yours.