Books....and it's really not close. The show though the first 4 seasons did a good job of adapting the story from the books, but really fell off the rails starting with season 5. The books have a much more elaborate and complex story to tell with Varys and Illyrio plotting to get a Blackfyre/Targaryen (young Griff) on the throne, the Dornish and the Iron Born trying to get to Daenerys to see who can marry her and seize Westeros, and the Stannis storyline...which is superb in the books, but completely mishandled in the show.
Don't get me wrong, I enjoy the show for what it is....an adaptation. However, if it's one or the other...the books easily win that discussion.
Keep in mind, if you venture into reading the novels, you're looking at roughly 1,100 pages, small print, each book. They're long, but will absolutely keep you engaged from start to finish.