I didn't watch the show (only the first episode) or read the books. But I have a way of thinking about this kind of stuff.
You should always watch the series/movies first, because normally books have more time to tell the story better, and also when you read it without images, you make portraits of characters or places based only on your imagination. When you watch the movie, series, you realize that its different from what you imagined, and you end up disappointed. Also they have to adapt the story.
So if you read the book first, you will probably get disappointed, what I see happening with almost everyone about most stories. But if you watch first you will like the movie/series, and will be interested in reading it, and you like that the book is even better, instead of liking the book and hating the movie/series because its different than what you expected.
Also when you watch the series, you will see the characters in the book as they appear in the series, you won't created their appearance based only on what you read.
Although Game of Thrones people like both, I always recommend watching first.