Hi, Andrew,
I have only walked the Vdlp via Ourense, but I have walked the
Camino Frances, so the only part I don't know is the part between Granja de Moreruela and Astorga. According to some people I have met along the way, that stretch is not anything to write home about. One guy I met described it to me as four days of penance, but my impression is that it's not difficult, and that there's just not much of anything along the route, so it's kind of boring. But it's just a few days.
I think you might want to leave the decision till you are actually close to it (the bifurcation is well marked in Granja de Moreruela), depending on what you may know about upcoming weather and the people you've been walking with. In my experience (but I've only walked this route once), everyone I met on the Vdlp took the Ourense alternative. Astorga to Santiago has a lot of those iconic places on the Camino -- Rabanal, Cruz de Hierro, O Cebreiro, but the Ourense alternative is quite beautiful as well. If you think you might be going back to walk the
Camino Frances, you could leave it all for the next time.
Though it may be a tough choice, there's no bad answer! Buen camino, Laurie