Most people had read the da vinci code first but if you have read neither, then you should read this first. You get to know the character or robert langdon better. I loved both books but i have to say that angels and demons was a much better read.
The problem with the movie versions is that there is so much to try to cram into two hours or so that you end up missing bits out. Its hard to know if you should read the book first and then see the movie or vice versa. I would recommend reading the book first as you cant enjoy the read if you know what is going to happen. After all, a movie lasts two hours but a good book can last for days.
Angels & Demons, is the first of Dan Brown's books that I have read, I loved all of it! I truly believe it is the best book I have ever read, it's simply impossible to put it down!! Worth every penny and more!! Fantastic!! I just hope the film does it justice (It's released on 15th May 2009!! Can't wait!!) 10 out of 10!! Brilliant!!!