Have both, loved both, although I loved the mythology on GoW, I would've loved to have a better in-depth story about the Umbran Witch's mythology if there ever was one, like how she got those huge-as.s summons and how they were dragged to hell, yeah they have cards/info but that's just shallow for me, but overall they are both awesome hack n slash games.
But to OP's question, I think Nintendo just wants to show fans that they can have mature games, don't know if they are competing though, but that's not an issue for me.