I voted the dads. That's just the way it is in our family.
But, If the baby is a boy and there is no other child in the mothers family to carry on the maiden name. Then the baby (boy) should pass on the family name.
I think that which ever side already has someone to pass the family name on then it should be the other side to carry on their last name.
Or if both do, Then father's name should be chosen