Margin
Margin is usually used to create a space between the element itself and its surround.
for example I use it when I'm building a navbar to make it sticks to the edges of the screen and for no white gap.
Padding
I usually use when I've an element inside a border, or something similar, and I want to decrease its size but at the time I want to keep the distance or the margin between the other elements around it.
So briefly, it's situational; it depends on what you are trying to do.
