There is basically one set of high speed tracks between Venice and Rome and Salerno and Naples. All the trains run on those tracks. There are 2 train operators, Trenitalia and Italotreno. Pick when you want to travel and buy tickets. Since train tickets are like airline tickets these days, pick when you want travel and buy tickets. The earlier you buy nonrefundable (or partly changeable) tickets, the lower the price will be.
You will need some buses on the Amalfi Coast to get around, and from Naples to the airport.