I'm not sure which way it will go. In the short run it will probably favor PS4 simply because of their install base. However in the long run I think I'd have to favor Switch. Dark Souls is really the type of game that appeals to old school gamers or people who like old school games. The Switch console really is much more for old school gamers than PS4 or XB1, especially if you look at the types of games released early 2017 for Switch. BotW, Bomberman, Street Figther, Puyo Puyo Tetris, etc.... Nintendo was really going for old school gamers when they launched the Switch.
I'm also inclined to think that Nintendo might give more marketing support to Dark Souls than Sony or Microsoft will. They have an amiibo with it, and before that January mini-direct they were acting like Dark Souls was a really big reveal.