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.
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.