I like the screw on discs. a lot of the snap on discs (not moon) look like a hub cap. You have to stay on top of the screws, they get loose and the disc has to come of to check your air pressure(unless you move stem to inside the wheel).
You haven't lived until you have chased a snap on across 6 lanes of traffic in LA.
If you race they must be removed to check lug nuts during inspection. I have started getting the dzus fasteners discs, they are so much easier to remove and replace. They are pricey but give you the real salt flat look.