Hi, Una,
I think you've got a great, but very tough, choice to make about whether to stay on the Norte or to dip down to Oviedo and go on the Primitivo. I've done both and would be hard pressed to say one is better than the other. I think the most likely thing is that you will be drawn to one or the other based on which way the people you've bonded with are going.
But here are a few differences.
Primitivo: Oviedo is a beautiful small city, and if you like pre-romanesque, the two sites on Mt. Naranco (one a palace, one a church) are incredible. Also, when you leave the Norte at Villaviciosa to head to Oviedo, you will have the option of a detour to another pre-romanesque gem at Valdedios. Stunning. Lugo is also a wow-zer city, IMO, walled, Roman ruins, etc.
Norte: CSJ albergue at Miraz is a wonderful place. Coastline west of the turn-off for the primitivo has some beautiful towns, particularly Cudillero and Luarca. The albergue at Tapia is right on the water, looking down on the coast from the headlands. A ***** view. And the coastal walk from Tapia to Ribadeo is one of the most beautiful ones on a very beautiful camino.
Either way, you can't go wrong. Buen camino, Laurie