Planning Trenitalia from Rome-Florence-Venice

Neither, book far ahead on the Italian railways website: http://www.trenitalia.com/tcom-en
April 2017 is not yet bookable, for a random day in February 2017, Rome to Florence booked today costs €24.90 second class.
Buying on the day at the station will cost €45 (so-called "base fare").
A Eurail Pass will be the most expensive option (work out the cost per day).

Second class is perfectly adequate, first class is unnecessary.
See here for lots of useful info about Italian trains: http://seat61.com/Italy-trains.htm

Reserving the tickets in advance gives a significant percentage discount, but as you can see from the examples above, the base/full fare in 2nd class is not prohibitive. Both of your routes cost about the same.

Second class is perfectly adequate, first class is unnecessary.

I agree. We always travel 2nd class in Italy and haven't felt as if we're missing anything.

For these routes choose the frecce trains, much faster and still very economical and you can choose your seats when you purchase tickets so you can all sit together.. Totally agree that 1st class unnecessary as 2nd is very comfortable, not that much difference except price is higher. Use the Trenitalia site in English, extremely user friendly.

In addition to what said above, I'd like to add that if you buy a discounted ticket (SuperEconomy and Economy) there are strict restrictions on train changes or refunds. In other words, you must be reasonably certain of your travel dates and times. If you miss the specific train you book, you lose your money.

The Base (full price) fare has no restrictions. You can buy the Base fare at any time, including as you walk to the station on the day of departure. Seats rarely, if ever, sell out, at most you have to wait for the next train 1/2 hour later.

Another thing. First and Second class is the name of the seat classes on FrecciaArgento trains.
On the FrecciaRossa trains, they have different classes, which I listed below.

I also listed the full fare prices (Base) for Florence-Venice, as an example. Rome-Florence has similar prices.

STANDARD (formerly 2nd Class) 49,00 €
PREMIUM 54,00 €
BUSINESS (formerly 1st Class) 64,00 €
BUSINESS AREA SILENZIO (NO TALK ON PHONES) 64,00 €
BUSINESS SALOTTINO (MINI CONFERENCE ROOMS) 84,00 €
EXECUTIVE 109,00 €

FrecciaArgento trains still has the old 1st and 2nd class designation. The prices are the same as Business and Standard as listed above. I agree that anything above Standard (2nd Class) is overkill.