Siena or Florence

You are so right. Both Florence and Siena are wonderful! I guess you need to decide which you would enjoy more. Would you prefer to share a town that you have been to before with someone you care about, or would you rather share a new adventure together? Either has its benefits. Florence is beautiful with all its art and architecture, and Siena really gives you insight into what it was like to live in a wall protected village. It would be wonderful to see them both, but I think you are wise in giving yourself more time to explore one or the other with the idea that this is just your first trip, and you will be back. I loved both Florence and Siena. That being said, and it is only my humble opinion, there is just something about the romantic quaintness of Siena that stole my heart.

Another thing to consider, train from Florence to Rome is 90 min. Train from Siena is double that.

I also loved both Florence and Siena, and I think you've already gotten good food for thought. I would only add that with only two nights, you can probably just scratch the surface in Florence (as you probably know from your previous visit). You would spend your limited time trying to squeeze in as much as possible, which is not an ideal ending for a trip. Two nights in Siena will give you a better opportunity to take in all it has to offer without feeling like you have to rush from one site to the next.

I don't think there's a wrong answer, but I'd lean toward Siena.

Florence over Siena!

Siena can be done in one full day, but Florence takes a minimum of 3 or 4 days to see.

If you only have two nights, go to Siena. Come back to see Florence, I have been twice and still want to go back.

You didn't say enough about your two personal interests, i.e. Renaissance Art vs. Gothic Art, architecture, expensive or cheap food, ancient monuments, church interiors, luxury hotels for a honeymoon, or what. Although I personally prefer Florence, I would say that visiting Siena is slightly more of an exotic city to wander through. Both are VERY crowded in summer, but you failed to give the month of the year.

I would enjoy an afternoon in Fiesole more than an afternoon in the Boboli Gardens ...

You didn't mention what month this is? If you are traveling in July (2) or August (16) around the Palio dates then go to Siena over Florence.

Robert, I'm not sure I agree with you. We happened to be near Siena last year's Palio day, and after investigation, we decided to visit elsewhere that day. Apparently you can do it like the New York Times Square New Year's Eve, prohibited from leaving or going to the bathroom for hours and hours, or buy a premium ticket (for hundreds of Euros) to sit in a chair by a cafe with food, drink and toilets. In any case, you are devoting 11AM -8PM to the racecourse, and then you want to walk around and see the contrada dinners. It requires a level of devotion ...

We were disappointed that, unlike New Years Eve, it was NOT on the TV's in our high-end country hotel, in Sinalunga.

Parking beside Siena was tough enough when we went a day or two later.

Either one is a wonderful place to be.

Maybe the logistics of getting to Rome for your flight will be what decides it.