Counter argument: corners make it easier to pour accurately. If it’s a thick soup, you’re going to get strips of soup stuck to the container between the spoon strokes either way and a corner can act as a soup staging area that makes it easier to get the soup out instead of just pushing it around. And a spatula would make quick work out of either shape.
Though note that I’m picturing a rectangular container with rounded edges.
Counter argument: corners make it easier to pour accurately. If it’s a thick soup, you’re going to get strips of soup stuck to the container between the spoon strokes either way and a corner can act as a soup staging area that makes it easier to get the soup out instead of just pushing it around. And a spatula would make quick work out of either shape.
Though note that I’m picturing a rectangular container with rounded edges.