Trenitalia's or Italo Trains

Trenitalia serves more of the country, since they are the national railway. Italo serves the main cities only. Both have basically the same price structure and train speed on routes where they compete. See discussion on these recent threads:

https://community.ricksteves.com/travel-forum/italy/trenitalia-or-italo-any-reason-one-might-be-better

https://community.ricksteves.com/travel-forum/italy/what-s-the-difference-between-trenitalia-and-italotreno-rail-services

kevin,

Whichever one has the timing and price that fit your schedule for a particular route. There is no "best" choice.

As Laura mentioned, Trenitalia is the national rail line (owned by the government) and therefore they offer much wider coverage over much of the country (except for a few private rail lines such as the Circumvesuviana in the south or TreNord in the north). Trenitalia offers both high speed and Regionale service so can reach the majority of towns and cities in Italy.

OTOH, Italo Treno is a private network that offers ONLY high speed service to selected cities in Italy, and they do not offer any Regionale service. In some cities they use different stations than those of Trenitalia, although this is slowly changing.

Which rail network to use will depend on where you're going. For example, if you're travelling to the Cinque Terre, Assisi, Orvieto, Siena, Lucca, Varenna or other towns that aren't on the high speed lines, you won't be able to use Italo as they don't go there. If you're travelling between two cities that are on the high speed lines, pick whichever rail network best fits your circumstances and has the best deals. I prefer to use Trentialia if a particular trip uses a combination of high speed and regional trains, as the timing is co-ordinated better between arrival of one train and departure of the next.

Not an equal choice. Go with price and schedule. The Italo trains are newer and some claim they are nicer. We have not see that much difference. Both systems service the same cities but not always the same train station so check carefully with the station being used.

Italo's rolling stock is generally newer, but Trenitalia's newest Frecciarossa 1000 trains have only been on the rails since mid-June, 2015.