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.
Nine Sols was clearly built on top of Hollow Knight, but my take on the combat is a bit different, Hollow Knight and Nine Sols would be like Dark Souls/Elden Ring (HK) and Sekiro (NS), one is focused on fast movement and dodges while the other is more focused on parry and counter attack mechanics.
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
+1 for Nine Sols. Also the setting of the game which the studio dubbed "Taopunk" for its combination of Taoism and Cyberpunk is very unique artistically. Also it had a very endearing cast of characters. As good as Hollow Knight is, story and character-wise it was very hard to follow for me personally.
