Just so you won't have an easy decision to make, I'll take the position that is polar opposite to Stuart's. I love Oviedo, and I have walked through there at least half a dozen times, and each time I try to take an extra day to visit. I think it is a beautiful small city with a very high level of civic pride and many wonderful public spaces for all to enjoy. Where else can you find a statue of Woodie Alan, a Botero statue of a mother and child, and a modern statue that appears to portray the human derriere (referred to in Oviedo as "El Culo." The large in-town park is very nice, but the park on the edge of town near the University is my favorite. The parque del Invierno, is a large expanse with many trails, snow capped mountains in the back, and green hills all around. And don't get me started on the pre-romanesque churches Stuart has already described, though I realize that is an acquired taste for many people. And as Stuart also mentions, the Cathedral, its treasures, and the statue of El Salvador have a lot of religious significance (aside from being quite visit-worthy in a secular vein as well).
When I walked the Norte in 2006, I did what you describe -- went down from Villaviciosa to Oviedo (there is a new albergue there on the way now, in Pola de Siero, which will make the journey much more pleasant). The benefit of taking that route is that you can take a tiny detour off to see another spectacular pre-romanesque church at Valdedios, in a beautiful valley setting.
The way from Villaviciosa to Oviedo is well marked and easy. But we had a terrible time getting from Oviedo back up to Aviles. Some comments here:
http://www.caminodesantiago.me/community/threads/norte-to-primitivo.19695/#post-147208
It is of course possible that the route has been better marked since then, maybe someone who has been there more recently can tell us.
Lots of fun decisions to make! Buen camino, Laurie
p.s. And Cudillero, about 3 km (?) off the Camino, is a touristy but great place to spend a night - its position in a cove on the ocean is amazing, lovely walks and views.