I've walked both and very much enjoyed both, and I think you should just plan to walk them both -- then making the decision for 2016 becomes easier.
But here are a couple of considerations
-- how many days do you have to walk this year? The Portugues is shorter from Lisbon than the Vdlp from Sevilla.
-- what time of year are you walking? I would avoid the Vdlp in summer or even September, starting in Sevilla, but from Lisbon to Santiago in summer you will not likely get those killer temperatures.
--do you want to visit a lot of monumental cities? If so, then Vdlp probably wins out, with Sevilla, Merida, Caceres, Salamanca, Zamora, and Ourense. The Portugues has its share of great stops -- Lisbon and Porto are both very enjoyable places to spend a few days, the old university town of Coimbra is very nice, the Roman ruins in Conimbriga are definitely worth a visit, the Convento to Cristo in Tomar is wonderful.
-- does asphalt really bother you? The amount of asphalt on the Portugues has been decreasing as the local groups are opening up new alternatives but there is more asphalt on the Portugues than the Vdlp, I would say.
-- are you monolingual? Many more people in Portugal speak English than in Spain, but in small towns in both countries you are likely to have to rely on hand gestures and google translate if you don't speak Spanish or Portuguese. (If you speak Spanish, you can make yourself understood in Portugal, but you will probably not understand a word of the response when they speak Portuguese back to you).
-- do you want a stretch along the coast? The Portuguese north of Porto has that option, but the Vdlp doesn't.
Here are ways in which they are pretty similar.
-- Fewer people than the Frances (though the Portugues on the last four or five days from Tui onward gets crowded, whereas I think there are rarely crowds of any sort on the Vdlp/Sanabres)
-- Not too much in the way of elevation gain, though the Portugues has one well known ascent north of Porto and the Vdlp gets hilly after Puebla de Sanabria
--Lots of small towns, agricultural areas, and very nice and helpful people (though I don't want to fall into stereotypes, I do think it's hard to find a country where you will be helped by more kind people than in Portugal).
No bad decision here, IMO! Buen Camino/Bom Caminho, whichever you decide! Laurie