I do believe that underpopulation would be the better option, in my eyes. It is terrible for me to see people fighting over basic needs like food, shelter, water, transport, land, etc. because there are just too many people and not enough provision.
However, I don't think it is our right to force people to limit the number of children they can have. It is their choice. We can educate them on why having fewer children is better.
Good luck with the contest, and thanks for the invite!