Stay in Siena or visit?

Just one opinion here but I really like Siena - even better than Florence - so my vote would be for two nights in Siena!

I'm with Sharon. But even if that were not the case, 3 hours would just not be worth it.

Well the biggest issue is my wife REALLY wants to go to San Gimignano and there really isn't a good wine tour from Siena. At least none that could find yet. I figured by the time we leave Rome in the morning and take two trains to Siena, half my day is gone anyway and our second day would have been an all day tour. There are way more option from Florence. Now if I could find someone to drive 6 of us around Tuscany for a day from Siena that would be ideal. Then we can hit whatever we want to.

We spent 5 nights in Florence and did a tour with WalkAboutFlorence - their BEST OF TUSCANY TOUR -- check it out online = they see Siena, SanG, Pisa and do a great lunch at a Tuscan Farm. We also did their CHIANTI WINE AND FOOD SAFARI TOUR. They were both totally worth it.

I agree with the others that a part day isn’t enough. We stayed for four nights. We toured around town on our own the first day, used a private guide on the second day and learned a lot of history of the contrade. Seeing inside some of the contrade Palio museums was fascinating. The last full day we did a wine tour with https://www.cooltours.it/. Ours was a shared tour but I know she does private tours.

My two cents is that changing accommodations and splitting your stay into 2 nights each is a big waste of time. You basically waste a full day in the transfer (check in/check out/unpack/repack, find the hotel, get oriented) and also from Rome to Siena it not as fast as Rome to Florence. I would consider splitting my stay only if I have at least 6 nights available for both. I would base myself in Florence and day trip from there, but I wouldn't split the stay if you only have few nights at your disposal.

If you visit just Siena for a day, you can do so by bus from Florence (75 min away). A day is plenty for Siena.
Then you could do a wine tour and a San Gimignano visit on another separate tour another day with a car (with a driver if you don't want to rent and drive yourself). With the time you save by not transferring hotels between the two cities, you can have two separate day trips. There are also tours that do San Gimignano+Siena+Wineries, but in my opinion it's too rushed.

Check out the Trenitalia website. I picked a random date in July. There’s a faster train (3plus hours) from Rome Termini, one change in Florence. Leaves at 0900 and arrives 12:30
I would choose Siena as well, but if you can’t find that special wine tour then ...

Well, we have found Siena to be more than a day trip. Sure, you can blast through the town in a day if you want, but doing at least 2 days/one night should be a good opportunity to explore most of it in depth. But having a cocktail and watching the sunset on Il Campo is worth spending the evening there. Just wandering through all the neighborhoods in the old town center is a joy. For dining, Salefino and Il Salotto are far enough off the more beaten paths. Bagoga is another great choice as well.

Also, keep in mind that San Gimignano can be reached by train to Poggibonsi, then bus. Probably 90 minutes. I also think there are buses direct from S.M. Novella to San Gimignano that are around the same price (€10 - one way) and about the same amount of time.

We took a day tour to Siena and San Gimignano from Florence. We were in Siena about 3.5 hours. I enjoyed it, but definitely wanted more time in Siena. I say spend a least one night there, or at least spend all day there.

I am presently planning a stay in Siena, and the wine tours offered by this company look good to me.

https://www.tuscanescapes.com/day-trips-siena

Why does your wife want to go to the packed with tourist bus loads San Gimignano? Siena is far superior. Do not do Siena as a day trip, rather stay several days. Maybe your wife can find a day tour to San Gimi? I have been there three times due to others wanting to visit there. Not high on my list of favorite places.

I would also vote for staying in Siena. We were there two nights with a Rick Steves tour and loved this charming town. On a subsequent trip, we stayed 2-3 more nights.

Have your wife look at Montepulciano on YouTube. She might want to switch over-touristed San G for that location, especially if you want a wine tour at the same time.

I very much agree with Roberto’s advice. We’ve stayed in Siena twice only because we love a particular hotel there, but we are not fans of Siena itself.

Siena is nice, we spend two nights there (toured for about a day and a half, that was plenty.
We had a car and spent a few hours at San Gimignano. It was nice to see, but there is not much to see there.

Now that we have the key information that wasn't in your OP, we know there isn't a rental car involved. I don't "love" having a rental car, but it is very useful in Tuscany - despite ZTLs and jammed parking lots all summer. So the short answer to your question is to ask a different question: "How to see San Gimignano without a car and without wasting too much non-touring time?"

I personally think that Siena is worth more than 3 hours, especially if the cathedral floor happens to be open to viewing on your visit week. Alas, even if you manage to get an all-Siena ticket that skips some lines, crowding and waiting are a significant time factor in both Siena and San G. San G is hilly enough and crowded enough that 3 hours are enough - not counting getting into and out of the town. I remember that parking in San G was hard to find a space, and a long uphill walk to the main drag. Honestly, heat and fatigue are important in summer Tuscany. I used to look down my nose at pool-hotel research there, but we jumped in our pool every day after touring!

Because we're talking Post-Covid, old posts about non-car options may not be as helpful as usual. And I've always had trouble finding really up-to-date web info in Italy.

Edit: While Siena is not remotely as big as Florence, it is much bigger than most of the other towns that people want to visit in Tuscany. In that sense, it is particularly suitable for one or more overnights (warnings about a group of 6 still apply.) But (say ... ) 2 nights in Siena are not like 2 nights in Cortona or Sant'Angelo in Colle or Monteriggioni, or even Pienza. I'm not saying any of those are must-sees. I'm suggesting that (medieval culture aside ... ) Siena is, urban life-wise, more like Florence than like a "Tuscan hill town." (I'm not romanticizing imagined rural life: Our travel style is NOT sitting at a table and watching locals go by! That's one reason we rented a car for our whole stay in Tuscany.)

I love Siena and would always highly recommend a stay there, but in this case I have to agree with Robert da Firenze. With 6 people traveling together, changing locations is going to be a production. It is easy to visit Siena and San G from Florence and wine tours abound. Try to give Siena a full day. Take the early rapid bus and the last rapid bus back (75 minutes each way). If you go on a Wednesday you can enjoy the morning market located right across the street from the bus station (according to friends in Siena, it is still in full swing), visit the Duomo, have a fantastic lunch at La Procuittoria, visit the City Hall (beautiful historical site), have a lovely apperitivo in the Campo and return to Florence happy and relaxed.

For individual driving tours around Tuscany try: www.toursaroundtuscany.com (they also do pick-ups and transfers).

Changing hotels doesn't have to be a waste of a day. When packing lite and with packing cubes it's a cinch to check out and take the train to Siena, drop off your luggage at the hotel and explore the city and have lunch. You were going there anyway so no time is wasted and the only hassle is having to take your luggage with you but at the same time you now save the 2 hour ride back to Florence. We enjoyed Sienna and the surrounding area very much so I think it's worth staying there for 2 nights. I suppose it also depends on where you are going next- for example Florence- Siena- Rome makes perfect sense (or Siena-Florence-Venice which is what I did).

We have spent many a stay in beautiful Siena over the years. Believe me I’m not recommending to “hit Siena as a day trip”. Spend two days and take it in. The TI has a great 3 hour city walking tour every day at 11:00 am that includes the spectacular duomo.
Wandering the back streets of the contrada trying to get lost is a must. Your wife will also appreciate the best shopping in all of Tuscany - ceramics, leather goods, shoes, foodie souvenirs, etc.
This is a full day at a minimum and you haven’t experienced their museums.
The evening spent on Il Campo with a wine or spritz is good people watching. Then you have some of the best Tuscan food in the land.

San Gimignano is a magical place.....before 10am and after 4pm. We would spend the night there but leave during the day as there were way too many bus tourists. Maybe now is a good time to go there before travelers are back to 100%. With that said, spending all your time in Siena would be well worth it! There is plenty to do and see...and eat! Also many photo opportunities of Tuscan rooftops and countryside.

Different people like different things.
Some people prefer to stay in Frenzy Firenze, which obviously offers more things to see and do, and day trip to Siena.
Others prefer the quaint medieval ambiance of a small magical town like Siena and would rather base themselves there and visit everything else, including Florence, from Siena,
Others would even prefer a smaller town like San Gimignano or even a secluded agriturismo hamlet in between, maybe in the Chianti Hills.
De gustibus non disputandum est.
That is a decision that only the OP and his/her party can make.
However changing hotels to move 50 miles from a place to the next for a party of six people is never quick. If one has many days at his disposal fine, but if not, in the time six people take to repack their luggage, clear the room, and meet in the lobby for checkout before leaving the hotel, one can easily travel from Florence to Siena (or viceversa depending on where one prefers to have a base).

Both Siena and Firenze have plenty to do for 4 days, though the later has the better art treasures. It would only be possible to visit Siena and San Gimignano in one day if you were on a tour or had a private driver. There's also the possibility of splitting up or a person staying back, if you're based in a fixed location. With six people, you're always as ready as the slowest person and that person can change from day to day. So meals, check out, and train/bus travel will be slower than with just a couple. I'd suggest sleeping in Firenze the entire time.
Editted to add: my wife and I stayed 3 nights in Firenze and 2 nights each in San Gimignano and Siena when we visited in 2014

We did a cruise ship tour to Volterra and San Gimiginano. Loved Volterra. Sam Gimiginano was packed with tourists. All the tourists were overwhelming. I have no desire to go back there.