Siena for longer term?

Will you have a car?

Which time of the year? With or without car?
If any time between April and October and with a car yes.
Without a car I’d stay in Florence or, even better, split the stay between Florence and Siena.
In winter I’d choose Florence regardless.

Plan is for Spring or Summer. Will rent a car as needed. Thank you for your insights.

Per Roberto, splitting time between Florence and Siena might be your best option.
Florence is a transportation hub, making it a pretty simple matter to hop on a bus or train for day trips around the region ... all the way to Venice for a couple of nights if you feel particularly ambitious. It's also only an hour and a half by fast train to Rome. Point being that you'll have plenty of options to keep you occupied if you base there for a couple of weeks.
Siena would be good for exploring the Tuscan hill towns and the Val D'Orcia by car - which is really the only way to do it any justice I think. If you opt to stay in the city center you could take advantage of the cheap parking available in the underground lot at the train station to park the vehicle. Costs 2 Euros per day and would keep you and the car well clear of the ZTL area, yet a short drive north to explore Chianti or south into some of the prettiest scenery in all of Italy.

Over the past few years, we have come to enjoy Siena as our home away from home. We stay six weeks to two months during each visit. During our time there we have made many friends, learned new things about the city and have immersed ourselves into in the culture of the neighborhoods. Many surrounding towns and villages are easily accessed by bus or train. We do use public transportation for the most part but have rented a car as needed. As stated in other comments Florence offers more options for places to visit faster using public transportation. We chose Siena because it has a more relaxed environment and still offers many options to enjoy during our visits.