We took the Trenitalia line to Rome and the Italo line to Venice. These are the 2 main lines for high speed trains in Italy. The tickets were much cheaper from Italo and overall we preferred the Italo train. The wifi connection didn't work on Trenitalia even after providing a credit card and creating an account.
The Italo train was free and easy to connect to. Wifi is essential when traveling to avoid high cost of roaming and be able to do some planning/research and communicate with loved ones back home when there is a bit of "down time." We also found that the communication was much better on the Italo line, all announcements were repeated in English and it was clear which stop we were at. On the Italo line there was no English communication and we nearly didn't get off the train in time. Our train from Florence to Venice was about 2 hours and cost $36 for 2 tickets... Seriously.
Bottom line- both trains offer quick and affordable transportation between major cities. if we go back we will likely choose whichever line offers the cheapest tickets however, if prices are comparable we would choose the Italo line every time.