Suppose we are agreed that for most purposes, "castellano" and "español" refer to pretty much the same language (though perhaps some may disagree---see Is there a difference between "español" and "castellano"? // ¿Hay alguna diferencia entre "español" y "castellano"?).
From reading that other question, I gather that in Spain, "castellano" is today preferred. One answer there also suggests that "Most places use español more but some places including Argentina and Uruguay use castellano more. Places that have a history of suffering under Spanish dominion also seem to prefer to avoid español."
So in which countries is one clearly preferred to the other?