Both are wonderful! Oviedo is a university town with a beautiful cathedral which houses the "crown jewels" of Asturias, the Camara Santa. This collection includes the Cross of Victory, the symbol of the principality of Asturias, and the Cross of the Angels, the symbol of the city of Oviedo. Both are covered in gold and jewels. Oviedo is also the Spanish home of Woody Allen (a little bit of trivia there)
And Leon....there is nothing more magnificent than the stained glass in the Leon cathedral (yes, I know there will be other opinions on this)! I could have stayed there all day watching the colors play across the statues and paintings inside. There is also an extensive religious art museum attached to the cathedral. I am not sure if they have tours in English (I was with my Spanish friend).