You usually want to throw as slow a disc as you can get away with. This allows you throw more directly at the target and they don't skip as far when they land on a hard surface (of course sometimes you want that). Another benefit of slower discs is that they tend not to kick as much to the side if they hit a tree. Fast drivers are best in an open area where accuracy isn't the main goal. They tend to be more forgiving when the throwers form is off which is harmful for someone who's still learning how to throw.