as I said above ambient lights = bad. You should never need to use an ambient light. It basically will flatten your entire scene out without doing much for it. What you need to remember is that ambient lights lights illuminate EVERYTHING equally, that's a bad thing.
Directional lights send light in from a direction (duh
!) and thus will not illuminate everything equally. They have proper shadow information and specular information etc. Take a look in the help docs for an explanation about the different types of light. But let me just reiterate ambient lights = bad!!
Technical Director - Framestore
Currently working on: Your Highness