300 (Movie) vs. 300 (Graphic Novel)
What are the differences!
Councilman and Oracle Subplot added to the movie
In the novel there's very brief mentions of this, if any, but in the movie they make an entire subplot of the councilman making reinforcement efforts and arguing with the Queen and the Oracle betraying Sparta in favor of the Persians. These changes do not do much to the story but they do help in exemplifying the relationship of Leonidas and the queen and making his ignorance towards the Oracle's warnings justified.
Leonidas portrayed slightly different
In the book Leonidas is portrayed as a very independent king who doesn't necessarily treat his army badly, but he does treat them as subjects. This differs from the movie where he treats his comrades like brothers in a way, and is also reliant on his queen (Who's role is greatly expanded in the movie) to make crucial decisions.
Special effects make the story more mythical
In the book the story is more "grounded" in the sense that this just seems like a very skilled group of 300 warriors. In the movie due to the special effects and casting it makes the 300 spartans seem like heroic super men and there is a dash of a magical element added to make it all seem more mythical in the end