Different modifiers will diffuse the light from the flash to varying amounts depending on the size of the diffuser or bounce card; it all varies on the size of the object diffusing the light.
From my understanding and experience, a bounce card will diffuse the light more than a diffuser on the flash. Bouncing the (unmodified) flash from a wall or ceiling will diffuse it even more as it has a larger area. The larger the surface area of the object diffusing the light, the more diffuse the light will be.
George.