One helps set the cast while the other encourages shape and shrinkage. The way you use your diffuser can completely change the final look of your curls.
Choosing Your Diffusing Style
Hover diffusing involves holding the dryer a few inches away from your hair to gently set the gel cast without disturbing the curl pattern. Pixie diffusing involves placing sections of hair into the diffuser bowl and pushing it up toward the scalp to encourage maximum shrinkage and bounce. Many stylists recommend starting with a hover diffuse to set the shape before finishing with pixie diffusing for volume.