You should always read the book first if at all possible. Granted, there will be times when you're seeing the movie in like 30 minutes and you don't have time to track the book down but reading the book (or whatever) first is ideal.
If you read the book after seeing the movie you'll hear Billy Crudup's voice in your head whenever you read Dr. Manhattan's dialogue. You'll hear Jackie Earle Haley's voice when you read Rorschach, etc. That's not necessarily a bad thing (and truth is, with trailers and everything you're probably already past the point where that could be avoided anyway) but you should strive for a "clean" read where you get to form your own take on the characters, the action etc.
A comic book is more visual but it's even more pronounced in a novel. If you read a novel without seeing the movie you'll actually imagine everything for yourself but once you've seen the movie it will probably be like reading a transcript of the movie. That's what you'll picture.