The steel slat design is more efficient for a border barrier. Because you can see through it. A normal wall people could be right behind it trying to breech and you would not see them. With the slat design you can see people on the other side of the wall from miles off. Think of it this way. If a border agent can travel at 50 mph in a patrol car and there's a brick wall he can know if someone is going to breech only if he happens to pass when they do so. He covers 50 *linear* miles an hour. With steel slats if he can see (on average) 3 miles past the barrier he can see 150 *square miles* of border per hour.
