train is more convenient for a day trip
Ciao
rosagold and welcome to our Forum!
Florence isn't that close to Milan so I am not sure a day trip would be enough time to travel there, see enough of the city and travel back all in a day.
In any case, most everyone recommends the train. First, because the only bus company I found that makes the trip doesn't go into Florence and drops you off on the outskirts - actually at the Firenze Certosa highway exit to be exact. You then have to catch a bus into the center of town and that would not only be inconvenient but make you waste more of your time. The ride from Milan to Florence takes about 3 hours and 40 minutes, while a bus ride from the Certosa exit to the center should take at least half an hour.
The bus obviously is cheap - I found it is 10 euros round trip from Milan to Florence. Check out the timetable for Sena bus at:
http://www.sena.it/ There are 4 stops in Milan so one of those could be convenient for you when catching the bus out of the city.
Much more convenient and expensive in any case is the
train, but at least you arrive right into the heart of Florence. That is pretty important when your time is limited. Train stations are always very central, you'll have to see whether the bus stops in Milan are convenient for you or not.
You can take advantage of Italy's new super fast trains, the Frecciarossa, which make the trip in 2 hours and 9 minutes. Second class fare is around 45 euros one-way.
There are other solutions, such as the InterCity trains, that cost less (27 euros one-way) and take longer (about 3 hours and 45 minutes on average). I guess at that point they are very similar to the bus. Check out Trenitalia's website for all timetables and fares.
http://www.ferroviedellostato.it The link to the English version is on the upper right. Search for "Milano" for departure and "Firenze S.M.N." for arrival. On the first results page, click on the "all solutions" button on the left side, middle of the page - otherwise it only shows some of the trains. At first I was only seeing the Frecciarossa times.
If you end up choosing to take the bus, remember to post back and tell us about your experience. I am sure you are not the only one looking into this as a possibility for moving around when visiting Italy!
Feel free to post any additional questions or doubts, I'll try to be as useful as I can

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