I found Nine Sols to be superior, much more engaging combat and more fluid as a whole, also with much darker story and more evil bosses.
Of course taste is subjective, it’s a great recommendation none the less.
I enjoyed both games but I found that with the exception of the last boss Nine Sols was a way easier game after you figure out how to parry effectively.
I also enjoyed the whimsical art style of HK a bit more than the (as said in a comment) "taopunk" style of NS, but that's purely subjective.
But if you enjoy metroidvanias both are great games that you should try.
For me it was the other way around. In Nine Sols I got immediately stuck in the boss (mini-boss?) fight against two enemies and the flying gal.
Hollow Knight was difficult at times (Coliseum of Fools, Radiance) but it never felt unfair. Some fights (Mantis Lords, Grimm) are among the best times I've had playing a game