First of all, I totally agree with what was said until now.
Please notice that Trenitalia has much more frequent high speed trains than italo, yet it generally is a more expensive company.
While Frecciarossa classes are executive (definitely a luxurious travel in a coach with only a tenth of leather seats and a selection of snacks and meals, only in this case they are haute cuisine servings), business, premium and standard (the only one without welcoming snacks, and it isn't necessarily the cheapest one), frecciargento only provides first (with snacks and slightly more room for your legs, yet I found more problems there with luggage space) and second class. If some passengers are aged below 26 or over 65, I suggest them to subscribe a cartafreccia (it's free), so that in case you didn't find anything more convenient, you can use the cartafreccia senior or young offer.
Italo (that is generally cheaper) will let you choose between Club executive, Prima, Comfort (w/o snacks) and *Smart" (w/o snacks; you can book cinema class instead of smart without any extra fees, but I don't know if movies in English are available).
I traveled with both companies choosing the two lowest classes, and I think italo is slightly more comfortable.
The most precious tip I can give you is to carefully check the departure and arrival stations you are willing to use.
Roma Tiburtina has a slightly different range of trains and it is served by the B subway line (on the Rebibbia only branch), but it is definitely not as crowded as Termini (served by both lines A and B), and there is no ticket check before boarding.
Venezia Mestre and Venezia Santa Lucia are in totally different parts of Venice, and while the first one is located in the land-part of Venice, the second one is built directly on the Canal Grande. But I guess you already know this.
A general tip from a student who needs to travel economically. Never try to purchase last-minute tickets for trains. Prices are insanely high. The best is to purchase tickets at least a month or two in advance, since they tend to cost less (and italo often gives promo codes via website home page that let you have up to 50% off even on already low-cost fares). Just to let yourself get a proper idea of how this works, the Frecciargento I take to get back home can vary from €19 to even €90 depending on how in advance I purchase my ticket.
I wish you the best luck anyway ;-)