I have the same experiences with both companies as Palal above. Both are OK, but in general I would prefer Italo.
To add: it's much more customer-friendly to book tickets on Italo compared to Trenitalia's shaky website. And Italo employees all speak English - which I can't say about Trenitalia!
The nice thing is that if booked a few months/weeks out, you can have great deals in 1st class which are cheaper than a full-fare 2nd class ticket.