Train from Rome to Venezia

Whichever one is cheaper when you're to commit to a date and time, or if one has a more convenient departure. There's no significant difference in price range, comfort, or speed. Each have several seating compartment options and discounts for advance reservation. Roma Termini is the main station, but some will also serve Roma Tiburtina, if that happens to be more convenient to your hotel (probably not).

Trains are trains. The idea of a "best" train doesn't work. Since the train frequency between most locations is very high you simply book the train that best fits your schedule. Trenitalia is the national railroad company of Italy and Italo is a relatively new private train company using trenitalia's track and station system. Italo primarily operates high speed trains and the pricing is similar between both companies. Personally do not think there is much difference or any reason to choice one over the other except if one schedule fits your needs better than the other. We travel most of the time in 2nd class unless we can get a very good deal (seldom) for first. Once you have ridden one train the above will make more sense.

PS There is a train (trenitalia) roughly every 30 mins between Rome and Venezia. Lots and lots of choices.

I prefer the colour of the .Italo train and their leather seats in second class.

Either will work just fine. Rail passes are not valid on .Italo. They are valid on Trenitalia but only with payment of an extra fee.

It is easy to get tickets on either network via their own apps.