is it better to provide a new demo for a script or an existing demo ?

Answer 1

If copy is provided with the job posting, then the client generally expects you to do a reading of their copy.  Some clients get very frustrated with generic demos, when they have requested a specific read.  You don't have to read the entire script provided, just enough to give them a good idea of what you can do.  Also, if you do a shorter read, you can do two or three versions to show variety.  I only provide existing demos if there is no copy provided.