If I'd read the book first, and then seen the movie afterwards, I would still have loved the movie (the voice cast is superb, and overall I feel like the movie does a great job capturing the feel of the book). But there's a certain amount of telling-not-showing in the movie that's handled more obliquely in the book, and I feel like the movie kind of loses something in that sense.

So, I would say try the book first. I have met a few folks who found the book's language and phrasing too stilted to read like contemporary high fantasy and not quite poetic enough to read like Tolkien. All of those folks liked the movie. YMMV.