No difference. The light has three wires going to it; a perminant live, and perminant earth and a switched earth.
When the light is set to come on when the door is open, the live feeds, passes through the bulb, and then earths through the circuit which has now been completed by the door open switch.
When you manually turn the light on, it completes the circuit between the perminant earth and live. In doing so it has no effect on the switched earth, the one which completes the circuit for the headlight relay.
However, the headlights will turn on whenever a door is opened, but to be honest i haven't found a bad thing about this yet.
Hope that all makes sense lol