Is this train ride worth the trip and money? What cities does the train pass through? Would it be better to fly?
Unless you plan to stop a couple days here and there I would certainly fly. It's much shorter and cheaper and if you don't plan to stop along the way the ride is not all that spectacular. Naples and Salerno are the only major cities the train stops by.
I am assuming you mean a private compartment on the night train? It depends on how well you can sleep on trains (stopping, hallway noise, lights from outside, jostling on and off the ferry). I would fly, unless I were taking the day trains and stopping in Napoli, Paestum or Tropea.