If I were to choose one over the other, in a flat choice, I would always choose the highly effective teacher, they would strive to do whatever it took to create gains in the children, and be humble enough to take on criticism.
On the other hand, if I were designing a school system, the best answer in my opinion, is that the question of how many students should be in a class should depend on the kids, and the circumstance. For young children, with many social-emotional concerns, having a smaller class so that you could manage the higher level of relationship and personal needs would trump the academic support. However, you could move that same group of smaller students to a larger group of students within that same year, if you effectively were dealing with their needs, as well as reduce to smaller groups, should you need to intervene more effectively. This should be a data driven decision based upon students' needs.