IMHO, I think that the interest in taking Cantabria will depend on which route will take P-L. If they choose to focus in Europe go for It. If they choose to focus in atlantic expansion and in some way don't care about the rest of Europe, leave Cantabria to Castille, and use them as a buffer Nation.I would actally prefer that no union with Castille happens, so that Portugal and Leon stay one country that has western part of Iberia, with eat being divided between Castille and Aragon. Also, I think that Portugal-Leon would think that it's in their interest to: a) take Cantabria from Castille and get territorial connection with Navarre and b) to have independent Navarre...
Just my 2 cents, of course, you are the author...