Hi, Ton,
Welcome to the forum, There are several GPS tracks of both caminos in wikiloc.com. But the numbers are very inconsistent, not so much for distance, but for elevation gain. (Just search for Camino Salvador Leon Oviedo and Camino Primitivo Oviedo Santiago)
For the Primitivo, one GPS track says total 11,505 m ascent and 11,556 m descent. Another says 9,347 ascent and 9430 descent.
For the Salvador, one says 4930 m ascent, 5658 m descent. Another says 2835 m ascent, 3352 m descent.
I think the lesson here is that though the GPS devices are pretty good with distances, their accuracy when it comes to elevation gain is much less so. But look at the wikiloc maps because they'll give you a good idea of the kind of terrain you'll be walking in.
In addition to thinking about the total elevation ascent and descent over the course of a camino, I think the way it's either spaced out or concentrated in a few stages is relevant. The Salvador has a couple of mountain stretches with a lot of up and down, and the rest is hilly and even some long stages of flat (like Campomanes to Mieres). The Primitivo just seems to roll along up and down all the way, with a couple of more pronounced ascents (to Puerto de Palo and another one after Fonsagrada) and descents (like down to the dam near Grandas de Salime).
I'm never able to answer the question -- which is your favorite Camino. But the fact that I'm heading back for my third Salvador/Primitivo this summer is a pretty good indicator of something! I think you would love it. Buen camino, Laurie