Any thoughts on whether it is better to purchase train tickets (Milan to Florence) via Trainline or through ItaliaRail directly? I have used Trainline in the UK but not beyond. Advice?
Neither are direct from the train company. They are both 3rd party resellers. Your choices for the high speed trains on that route are Trenitalia (https://www.trenitalia.com/it.html) or Italo (italotreno.com) I've never had a problem buying tickets online from Trenitalia.
The 2 you listed are third party resellers
Purchase from the actual train companies linked above
Both Trenitalia and ItaloTreno have easy to use apps
Trenitalia is the official train company and runs fast and regional trains
ItaloTreno is a private company that only runs fast trains on select routes
It’s usually best to buy directly from the train operator for tickets in advance, although I find buying last minute tickets (think arriving from transatlantic flight and waiting to purchase a ticket until you definitely know your arrival time) from Trainline to be be very convenient. Their app. Is so user friendly and I can book a ticket in mere seconds. Some on here will point out additional fees they (and other third party sites) charge but in my experience the fee difference is hardly noticeable.