I read the two out of order, but I don't think it lessened my enjoyment. The Princess and the Goblin has a slightly stronger plot than The Princess and Curdie, I think, but it lacks the saturation of Christian Analogy that I found in the latter. 
 Both will get at least a second time through. MacDonald's stories can seem light, and even childish on the surface, but I'm finding that he hides layers of truth and insight into the human condition beneath the fairytale facade.