The line from Santander to Gijon and Ferrol is indeed very picturesque and coastal and certainly is well worth a trip but it does not compare to the Santander, Bilbao, Leon, Oviedo, Santander triangle.
IMHO the triangle is superior because it has vastly more variable spectacular mountainous and seascape scenery, does not require a return duplicate trip along the some route, includes the unbelievable but soon to be replaced Pajares pass and provides one of the great railway journeys using the historic 200 mile La Robla railway over the Cordillera Cantabrica. Additionally it is a more compact route and can sometimes be completed in 2 days, but 3 would be better. For the more nerdy, including myself, it also allows a proper loco hauled train to be used on the La Robla section on certain days of the week. Finally Ferrol is a bit grot.
Having said that all of FEVE is well worth experiencing, even Ferrol.