Generally speaking, I know to use translucent for leaves and SSS for skin, but why is this? Physically speaking, how do these shaders refract/scatter rays differently? Why does translucent have no sliders or options? If anyone could sketch up some rough optical diagrams, that would be absolutely awesome! Any responses are appreciated :)
Translucency allows light to pass through an object. Using only a translucent shader will leave an object dark on the side receiving light with the other sides being lit.
Sub Surface Scattering allows light to penetrate the surface and be distorted before bouncing back out. Each colour component disperses differently, leaving areas with a different colour than others. Thin areas allow light to pass through to light the opposite side before being bounced back out, these thin areas give a translucent appearance.