From the viewpoint of a reactor moderator, is it better for a reactor moderator to have a large slowing down decrement or a small one? Suppose that the energy of a thermal neutron in a reactor core...

From the viewpoint of a reactor moderator, is it better for a reactor moderator to have a large slowing down decrement or a small one?

Suppose that the energy of a thermal neutron in a reactor core happens to be exactly 0.025 eV. Is this the average energy of the neutron or its most probable energy?