The one hour flight time is a bit deceiving. Once you factor in the entire travel time the train is quick. Flying requires a trip to the airport, time before the flight and then transport from Treviso to Venice. I'd stick with the train, regardless if you decide to stop in Florence along the way. Check train schedule at Trenitalia - most trains stop at Mestre & Venice S. Lucia.

If you stop in Florence you will need two tickets - one from Rome to Florence and another from Florence to Venice. You cannot buy one ticket and stop in a city along your way.