Hi, we are a group of 8 going to Italy in Oct 2019. We'll be arriving in Rome on a Sunday. I read on another thread that it is better and cheaper to take the regional train from the airport to Tiburtina. How much time should we allow between the train arriving at Tiburtina and the train departing to Florence (the next leg of our journey)?
scythian is spot on: if you can make the timing work, taking a train directly from Fiumicino airport to Florence - without having to go into Rome - is the way to go.
But either way you choose to do it, there's always a risk in pre-purchasing tickets for longer-distance "fast " trains on arrival day., Flight delays and cancellations are not unheard of, and you can be out the price of your tickets (depending on what type you purchased*) if you miss the train you have seat reservations for.
(*) For instance, Italo (a private rail service) offers a flex ticket that can be changed for no charge up to 3 minutes before a scheduled train departs, and up to 2 hours after. They don't serve the airport but do have service from Tiburtina to Florence.
Taking the regional train to Tiburtina vs Leonardo Express to Termini saves 6 EUR per person, as that is the price difference in the local trains, 14 for the LE and 8 for the Regionale. Taking one of the direct Freccia trains is the same price as taking the LE to Termini and changing to a Freccia to Firenze. The direct trains do call at Termini, but not having to transfer saves the hassle and about 15 minutes of transfer time.
I would feel much safer to travel to Tiburtina/Termini (whatsoever will be available) first, do a layover and then going to Florence. You would never predict either your flight will be delayed or not. There's a frequent connection between Rome and Florence at this point.
Frankly, with a party of 8, I'd use the Leonardo Express into Rome. While this is a larger station, it's much more straightforward in finding the connecting tracks. You'll also have a longer layover here.
With Tiburtina you have to go up to the main area then down to the track level which in some instances in the tunnel area can be confusing for first time visitors to this station.
Both tickets can be purchased at the airport station. When you arrive.
The walk between tracks at Tiburtina can be long and the regional trains sometimes run late, so allow at least a half hour between trains. With the new secured area at Termini and the Leonardo Express generally running on schedule, I think the change there is better. That said, I second the recommendation to take one of the direct trains if it fits your schedule.