IMO, there are "Venice people' and "Florence people."

I'm a Florence person, and I'm always puzzled when it's described as overrated. Crowded? Yes, usually. But overrated? Florence has the greatest concentration of universally renowned works of art in the world. The historical center remains much as it has been for centuries. And, after Rome and Bologna, it has some of the best food and wine in Italy.

I like Florence and Venice. If this is your first visit then you should give time to both. The structural aspects of the two cities (lake based and river valley bottom) makes them very different. One a maritime republic looks always to the sea but the trees that make up the islands made land very expensive so the city is a place of narrow alley ways. The other, a land based force, with more space is a city of open piazze.

Venice is very much the better at the end of the day or early morning when the fat boat people have waddled off. Florence has a similar problem with coaches but generally there are always quiet places to admire the architecture. The "drum" of luggage can be heard in both.

Do consider visiting also la Toscana area. Siena, for example

I would stop over for a few hours in Padua if I visit Venice. On photos it looks every bit as beautiful as the other Italian cities but it doesn't have as many tourist crowds.
The trains and I assume buses from Florence to Venice pass through/by Padua anyway.
Some say Ferrara might be worth it as well, it's also on the way between Florence and Venice.

Agree with museums and food which I mentioned. The city itself IMO is not worth the hype. There are many more beautiful and walkable cities, especially Venice.

Disagree with you there. Firenze is a wonderful walking city, and being that I have many friends there, natives and expats alike, who live both in the city center and the outskirts, they all walk to their homes from the center. One can’t compare Venezia to Firenze as walking cities because they couldn’t more different. @OP-If this were my trip I’d do Florence and Venice and leave Rome for the next trip. You’ll need at least 4 days in Florence to see all of the relevant art and foodie places. Venice will need at least 3-4, maybe 5 days. You WILL get lost. It’s all in the fun. Enjoy!

The other way to see Venice is from the water, the waterbuses offer a great view, but so too would any boat, looking on the ity from the crowded alleys can be depressing.

In the summer, the vaporetti are extremely crowded. The last time I was there in the summer, I felt the vaporetto we were on was actually dangerously overcrowded. It wasn't a great way to see the city.

I've always felt Venice was a delightful city to walk around. Once you leave overly well-beaten trail from the train station to the Rialto Bridge to St. Mark's Square, you can find peace and tranquility.

Last time we were in Venice, some few years ago, besides the vaporetti, they were running some kind of express tourist vaporetti that one could buy a pass for, and they were much less crowded. I seem to remember they did away with those because tourists were not wanting to pay the extra dollars. But at the time it was great for us! We nearly had the boat to ourselves most days.

I also have a great interest in architecture and churches. I spent a week once in Venice just to finally have time to see many of the churches and the art work hanging in the spots they were painted for and to hear music played in the spaces it was written for. There are dozens and dozens of churches and big Basilicas which you will automatically include, plus those in which you have a personal interest. There are a few that IMHO seem overlooked.

In Venice,
I highly recommend one of the most special churches which few people have time for. It is the Santa Maria dei Miracoli. Initially, it was not easy for us to find, but so worth it. I won’t try to describe it. Look at some pictures and read about the cleaning done in the late 90s.
I also recommend trying to see one of the synagogues. Tickets include a brief tour and the history of Jews in Venice.
In seeing San Marco, try not to look at the Basilica right off. Instead, walk away to the opposite end of the piazza, then turn for your first look. If you love churches, it will make you weep.
There are concerts in many of the churches, some short (30 minutes or less) and some even free. Check to see where you can stop in for a few minutes. If they have something short, The church of Santa Maria Della Salute is great, unless you want something longer with a ticket.
There is so much to Venice, I would not leave it to go to Padua unless the paintings by Giotto are of really special interest to you. I would instead go to one or more of the islands (I prefer Burano) in the Lagoon. IMHO, that gives a better picture of Venice.

In Florence, there are so many interesting churches besides the Duomo, but do not miss The Basilica di Santa Croce. It is a minor Basilica, and the interior is so unique and absolutely wonderful. The history of it is also fascinating.
Before going, try to read Brunelleschi’s Dome. Understanding the history, the construction and the connection of the Duomo to the Pantheon in Rome will make it more meaningful.
You can’t miss Ponte Vecchio, but walking across does not give the real charm. Walk down river and look up at the outside! It is Florence!

The OP has not posted since January 31st (same day as opening this thread). I hope they return soon to see the great advice they have been given.