It makes no difference. Just make sure that the train departs from SMN Station in Florence (since a handful of AV trains depart from Campo di Marte).
If you go to the Trenitalia website, you'll see a detailed description of both AV train categories, and you'll be able to discern the technical distinctions. However, these are distinctions without a difference in terms of the travel experience between Florence and Rome.
In case you didn't notice, the Frecciargento is actually two minutes FASTER than the Frecciarossa from Florence SMN to Roma Termini!!!! (it is a slightly shorter train).
Choose the one that leaves Florence at the time you want to travel.
Thx for the help!
Ok, I'm trying to call this company to make a reservation, but can't get through. I'm calling from US and the number they have listed on the site 0039 06 68475475 isn't working. Does anybody know of another?
From the US, you have to call 011 39 etc.
You won't be able to book online with an American credit card.
Seat numbering on the ES trains is abstruse.
They keep trying to give me Coach: 11 Seat: 22 , 28. Are these next to each other?
Stop worrying about this, as it is highly unlikely that the Trenitalia site will accept your US credit card. Just buy your tickets at a station when you get to Italy.
If you are having any troubles booking we managed to use a Canadian Mastercard with no problem on http://www.seat61.com/Italy-trains.htm (which has fantasic explanations) and from there follow the link to http://www.trenitalia.com/ which is where we actually purchased our tickets. All seating is pre-assigned but they try to get you as close as possible. It works for Canadians but may not for Americans. Don't even think about using American Express, it's the worst possible card to travel with.