Not having touched Silksong yet, I'd recommend playing the Hollow Knight first. It holds up really well and is absolutely worth a play through. I think sometimes a sequel releases and it makes the original feel hard to go back to, if it's added new features or quality of life things. If I were in your position, I think having the "uncontaminated" experience of Hollow Knight is worth it.

That is, unless you really want to feel like you're part of the conversation these first few weeks after Silksong comes out.