Trenitalia or Italo — which should I take?

Take whichever leaves when you want to leave; on the routes they share they're equally good. Italo is the private competitor, often a little cheaper and slightly nicer inside; Trenitalia runs far more routes, including everything regional, so it's the only option for most smaller places. Both are punctual by American standards. Buy in whichever app you like, book two to four weeks out for the best fares on high-speed, and don't pay a third-party reseller a booking fee for the privilege of doing what you can do directly in five minutes.