Hi, gianlucca,
This topic has come up before, and the bottom line is that it is an extremely tough decision, and either choice is excellent. So that's no help, I know.
I have walked both of these routes. When I walked the Norte from Irun, I stayed on the Camino del Norte the whole way, staying on the coast to Ribadeo, and then going inland through Galicia to Santiago. When I walked the Primitivo, I started in Leon and first walked the Salvador to Oviedo and then continued on the Primitivo.
Here are a couple of things that might make a difference to you. If you are a true Camino addict, and if it's likely that you are going to return again and again to the camino, I'd say stick to the norte for the entire trip, and leave the Primitivo for another year, maybe combining it with the Salvador. I say that because I think most people are very unlikely to come back to "finish up" the norte if they detour south to Oviedo from Villaviciosa.
But if you don't plan a repeat performance on the camino, then I'd say leave it up to the moment at which you have to make the turn-off, and then just go with the flow -- by then you will have your friends or "camino family", and you will see which way they are planning to go.
In terms of a straight up comparison, as betwen the Primitivo or the Norte after Gijon, the main obvious difference is that you leave the coast for the Primitivo. But by then you will have had a lot of coast if you've walked from Irun. So, I guess this is a long-winded way of saying they are both extremely beautiful, they both have good pilgrim infrastructure, and they both have enough pilgrims walking to create the camino sense of community. Toss a coin. Buen camino, Laurie