Well, supposedly, Fast is the speed that has the "speed of the game" accurate in that players run at their realistic 40 times (faster guys will run 4.2 or under while slow guys will be more than a second slower). So players will move about at their real speeds and then the flow of the game is supposedly the most NFL like.
In terms of gameplay and "how it looks on tv", though I think slow is closest to that, though Very slow is really nice with the animations.