SnS doesnt really do enough to make it much different from dark souls, but i think that was the intent. HK i think introduces enough mechanics and makes enough use of them to make it stand out as its own unique game.
As far as the story, its like the SnS devs pretty much just said>fuck it, replace souls with salt and print it
Whereas HK uses its NPCs enough to expand more on its lore compared to SnS
I really like both tho and and think both are worth playing.