It really doesn't matter, just know there may be some differences one way or the other. For some people, watching an adaptation first can be really helpful in their understanding of the text itself.

Many prefer to read the book first, but I notice that sometimes people do that just so they can feel some sense of superiority over those who haven't read the book or didn't read the book first (full disclosure, I have been that person and still sometimes am, but I realize it's a dick move and it doesn't really matter in the long run.) Take in a story whenever and however you want to take it in!