Gouraud Shading is effective for shading surfaces which reflect
light diffusely. Specular reflections can be modelled using Gouraud
Shading, but the shape of the specluar highlight produced is
dependent on the relative positions of the underlying polygons. The
advantage of Gouraud shading is that it is computationally the less
expensive of the two model, only requring the evaluation of the
intensity equation at the polygon vertices, and then bilinear
interpolation of these values for each pixels.
Phong Shading produces highlights which are much less dependent
on the underlying polygons. However, more calculation are required,
involving the interpolation of the surface normal and the
evaluation of the intensity function for each pixel.