specularity is a really a fake since it's not actually relecting light and is used to simulate reflectiveness ,where as reflectivity is the real deal and is actually relecting light .
The main difference between them in a 3d package rendertime, since specularity or specular colour is not actually relecting but just changing the colour of the surface to simlate relecting light there is no calculation to be done so therefore a lot quicker to render.