Taking the train from Florence to Venice June 8

You only have two choices.
www.trenitalia.com
Or
www.italotreno.it
The full (base) price for a 2nd class (Smart class on Italotreno) ticket is about €45 with both companies, although discounts are available if you purchase in advance (be alert that discounted tickets aren't refundable if you miss the train you book). The websites above will give you the price options.
2nd class is more than adequate, certainly more comfortable than business class on an airplane. Luggage goes on the overhead rack above your seat, or if you travel with oversize steamship size trunks there is a compartment at each end of every train car.

Just looked on Trenitalia, you can get economy tickets for €29 on most of the routes for the 8th of June. With this ticket you can change the date and hour once up to departure.
I have purchased many tickets on-line. I have notified my credit card company and had figured out what time I wanted to depart.

I agree with Robert, 2nd class on this train is fine.

I would recommend if you are traveling with others that you select your seats rather than automatic seat assignment, that way you are sure you sitting with them.

The trains are a great way to travel!

Roberto is correct about the booking of trains. If you book for the economy price, you cannot change the date or time of the departure. Just wanted to make sure you get the correct information.

super economy tickets have no refund or changes but economy tickets allow one change. As I mentioned in my above post, there are economy tickets available which are less than base fare tickets.

from the Trenitalia website
"You can change date and hour once up to departure, choosing a journey on same category of train, according to the ticket change procedure and paying the difference between the full price (Base) applied to the train used and the sum already paid."
http://www.trenitalia.com/cms/v/index.jsp?vgnextoid=3d72bd6bc1abf310VgnVCM1000008916f90aRCRD

You second statement, Holly, is the correct one. Your first statement implied that you could change it without an additional charge. Just remember all tickets are dead, if the train leaves the station. In old days not so long ago you have 90 minutes to change the ticket if you missed the train. Not so with the new rules.

I have been happy with second class on both brands of train. Luggage racks are also equivalent. You can decide whether you are ready to lock in travel date and time now, possibly for 29 euros per person, or to pay 45 euros on shorter notice, or even on the day of travel.