Florence and Venice for one week, how long in each?

With four days , or is it four nights which equals three days? Take the train from Rome. Yes, from Florence you could visit beautiful Siena.
Skip Pisa. Skip the Cinque Terre. You do not have enough time. Book a vacation to Tuscany another time and rent a car to see these places.
Florence has so much to see and if you only have three full days to sightsee there, just visit Siena. Then you would only have two days to see Florence’s treasures. Book entrance ahead for L’Accademia to see Michelangelo’s “David.” and for the Uffizi. Allow time for walking around this charming city.
And stay put in Venice if you only have three full days( four nights?)

Florence 4 nights (3.5 days). You can use a day to day trip to Siena. Cinque Terre is too far and in April the weather may be iffy. Lucca+Pisa can be a day trip, but you have to choose between that and Siena, and I think Siena is more worthy.

Venice 3 nights (2.5 days). You could even venture to the islands of Murano and Burano for a quick visit, maybe the afternoon of the day you arrive after checking into the hotel. At the end of April the sun sets after 8pm in Venice, so I think you have enough time to visit both islands in half day. Then the next two days you can fully devote to Venice.

Here’s a website to check fast trains from Rome (Roma Termini is the station you want) and Florence (Firenze SM Novella is the station you’ll arrive and depart from). Venice is Venezia Santa Lucia (end of line): https://www.trenitalia.com/en.
I agree with the others. You need a minimum of four nights in Florence that should include a day trip to Siena because you can take a direct bus (1h 15m) there and back. You don’t have time for any other day trips.
If you sleep in Venice for three nights, I would stay in Venice and not visit Murano or Burrano, you don't have time. Venice deserves two full days.

We stayed 4 nights in Florence, and one of the days we rode the bus to Siena, rented a car, and then drove around Tuscany for the day, then took the bus back to Florence. It was a full day, but very doable.

To me there is much more to see in Florence, Tuscany, etc, than Venice. If you want to go to Cinque Terra, I suggest a side trip to Lucca. It is a magical town and you can get to Pisa and Cinque Terra easily by train.

For us, staying several nights in a town means an Airbnb or VRBO so we can spread out and check out the local supermarkets to make our breakfast and lunches.

Is this "in addition to" or "instead of" the itineraries you mention in your other posts?

How many nights are you planning?

From where do you fly in /out?

Thanks everyone. We will be in Rome and the Amalfi Coast area the week before with our adult children then they fly back (out of Rome) and we are planning to spend our second week in Florence and Venice (we fly back home from Venice). So it will be 4 nights in Florence (3 full days with 2 half day on the front and back and 3 nights in Venice (2 full days, 1 half day on the front). We plan to stay put in Venice but now sounds like a trip to Siena is doable from Florence. Any hotel recommendations in central locations in both areas (under 300 euro per night)? TIA

In June I stayed at the B & B Alle Due Porte in Siena, close to the Piazza and Cathedral. Hundreds of years old building with tasteful updates. The host was very helpful and accommodating.

4 and 3 is good. No need to take the high speed trains, but nothing wrong with it, depends on your budget. High speed trains are fun to take.

I recommend Tours by Roberto for Siena.... we had been to Siena 2 twice before and our 3rd visit was with Roberto.... holy cow best tour ever.

Having been to Italy several times over many years, I have welcomed the high-speed trains. On an itinerary of just a few days, switching cities and lodging move out/move in can consume many valuable hours — sometimes most of a day — that could have been used for enjoying the destination. So for example a couple months ago, I was very happy to be able to get from Florence to Venice in only 2 hours 15 minutes on the 200+km/hr Italo train.

Overall 4 days in Florence and 3 in Venice sounds fine. If you are not renting a car, you can get to several regional towns from Florence on the train or bus. Siena is a worthwhile day trip from Florence. With 4 days in Florence you should also explore the other side of the Arno river.

Venice can be polarizing. Some love it, some hate it. The ones who hate it get tired and worn down by the crowds in the touristy center and around the major sights in the middle of the day. The ones who love it (including us) spend a fair bit of time out in the neighborhoods, back streets, and other quieter islands, sitting at a neighborhood bar away from the center looking at the water, or wandering through random piazzas and canals at night after the day tripper crowds have left. With 3 days, if you can get the major sites out of the way in a day or two, consider spending the rest of the time exploring less intense areas, getting to understand how a marshy lagoon turned into a city quite unlike anywhere else on earth.

Last year we stayed at the Hotel Paganelli in Venice and were pleased. It's very close to St Mark Square. We also stayed at B&B Casa Camilla in Padua. Our host at Casa Camilla was amazing. He gave us a map of Padua with important sites marked. He told us that he didn't understand why more tourists didn't stay in Padua instead of Venice. The local train is only about 5 euros and took maybe 30 minutes. After staying there, I really understood, and might try that sometime. However, waking up early in Venice and roaming the streets early with no crowds is pretty amazing.

Thank you everyone!