The hook size will change the final look of the headband. A larger hook will make looser, more open stitches and a smaller hook will give tighter stitches. A larger hook is used in projects like this crocheted headband and items like blankets. Smaller hooks are used in the (currently very popular) amigurumi. Of course, you can use whatever sized hook you like best. However, if you don't use the J hook like I did, you'll still end up with a headband, it just won't look the same as mine.