Should I use the screw terminal or the holds in the back of the electrical outlet to attach the wires?

Technically, you can use either one. But wires “backstabbed” into these holes (as it is called) are more likely to fall out, which could start a fire. Most electricians recommend attaching the wires to the screw terminals, with the wire wrapped around the screw clockwise, to ensure a solid wired connection when the screw is tightened.