+atk/-def vs +hp/-spd: which Nowi to promote?

That's a pretty tough choice, though if her only job is to check red swords then I'd go with +hp/-spd with TA/QR build. She gets doubled by most of them anyways, though the -spd makes it so she gets insta quadded by alI brave bow users and hit with luna, so be careful about those. I believe she one shots both ayra and sigurd unless they have some fortify buffs going on, though you can check for yourself. The +hp also makes her a good candidate for panic ploy, whether as her C skill or the seal.