Mike,
I know what you mean about choices. Here are some random comparisons between the two routes
-- in both you'll have some sections that aren't so pilgrim-ready. The Invierno has a couple of wonderful albergues, but a lot is in pensiones. Same is true for the Vadiniense. But if you want an unbroken line of good pilgrim infrastructure, the Madrid-Leon-Oviedo-Santiago combination would be hard to beat.
--on the VAdiniense-Salvador-Primitivo, you are alone at the beginning, but from Oviedo onward you will surely find your own wonderful camino family and will have some real albergue standouts (San Juan de Villapanada and Bodenaya would be at the top of my list).
-- on the Madrid-Invierno, you will essentially be alone the whole time (of course numbers are growing), except between Sahagun and Ponferrada (which is on the Frances), and from Laxe to Santiago (the Invierno merges with the Vdlp in Laxe, an albergue a few kms outside of the city of Lalin)
--Madrid to Sahagun is essentially flat, except the lovely day from Cercedilla over the pass of Fonfria. But the pilgrim infrastructure is astonishingly good. I did walk with two Spanish pilgrims for about 6 days but after that I was without exception the only person in every albergue I slept in. I'm a 62 year old woman, never felt anxious or creeped out.
--For mountains, the Vadiniense/Salvador/Primitivo is hard to beat. There is more road walking on the Norte/Vadiniense segments than on any of the others but the scenery is pretty incredible and the off-road stages (much of Potes to Fuente De and Fuente De through the Picos) are spectacular, not to mention the Buiza-Pajares stages on the Salvador, which are also incredible.
If you think you will be back to do both of these two alternatives, I think the pairings I used are sensible, but if you think you will only do one, maybe madrid-leon-Oviedo-Santiago is the best in terms of totality of off-road walking, pilgrim infrastructure, and the strength of the pilgrim community on the Primitivo from Oviedo to Santiago.
There's no bad decision here, that's for sure! Laurie