i think the two different styles were designed to reduce deposits left on the injectors after you shut the car off. ball style just puls a sphere-like(sphereacle?) surface onto a seat of of holes, and the fuel flows past this and through the holes in a wider misty-er pattern, allowing for better air/fuel mixture. the disc style has two flat surfaces, one with holes in it. when the injector isnt energized, the two surfaces are flat against each other. when the injector IS engerized, the surfaces seperate, allowing fuel to get through. then surfaces squeeze together again, spraying out fuel.

or something like that..