Well, I can’t give a recommendation, but I can give you my memories of the two different options.
San Vicente to Cistierna. San Vicente to Potes is pleasant, a hill or two, no mountains yet. Potes to Fuente De is spectacular. And the day from Puente Dé through the Senda da Remoña is really beautiful. It’s very easy to navigate, because there were once plans to pave a road through there. Fortunately those plans were ditched, but I think a vehicle can easily get through on it. Once you get through the pass, it is a lot of asphalt down to Portilla de la Reina. From Portilla to Riano to Ventasierra to Cistierna (it’s got an “n”, not a “rr”, not that that’s a big deal but it might impede searching) is pretty flat, around a reservoir (reservoirs usually depress me, as do the towns that were flooded and then recreated up higher). The sections along the Esla River are pleasant but no mountains. That’s about 7 or 8 days total, I think.
Aguilar to Cistierna is shorter. 4 long days. Aguilar to Cervera is also pleasant, along a river, no mountains, and also rings a reservoir (less depressing to me for some reason). Cervera to Guardo is long and there is a small ascent at the beginning, but most of it is weaving through agricultural fields with the Monte Palentina in the background. I loved that day. From Guardo to Puente Almuhey, there is a mountain option, which is great- Up to Caminayo (you can sleep in the social center if you want to break the stage up), but the last kms into Guardo are along the road. Then from Puente Almuhey to Cistierna is a much nicer entrance into Cistierna than on the Vadiniense, because you go up to the La Velilla sanctuary (with Santiago statue) and from there it’s a very nice high elevation walk (I don’t know what the cutoff between hills and mountains is) and then down into Cistierna.
A couple of caveats — they have re-routed the first few days of the Lebaniego/Vadiniense, making it longer I think but less asphalt. A forum member was recently there and posted live, so you’ve probably seen @seanmcauley’s threads. My impressionistic comparison would be that the most spectacular mountains are the Picos de Europa on the Vadiniense, but the Olvidado has overall more beauty packed into its three days from Aguilar to Cistierna than the Vadiniense has packed into its 8 days, because from the pass to Portilla to Riano to Ventasierra to Cistierna has a lot along the road (with some lovely exceptions) and very little elevation.
Hoping this will give you some starting point basis for comparison, happy to continue the conversation! Buen camino, Laurie