Home base in Florence or Umbrian town?

Umbria and Tuscany are two different regions, and transportation across regions can be time-consuming unless you have a car.

You could base three nights in Firenze and three nights somewhere in Umbria (Assisi would be good). From Firenze you could daytrip to Siena, from Assisi you can get to several beautiful Umbrian hill towns (Spello, Todi and Trevi, for example).

Zoe has it right: Day-trips often mean buses, and buses tend to stick within Umbria or within Tuscany. Also be aware that there are few inter-city public buses moving on Sundays, so plan your Sundays carefully to avoid that particular monkey wrench. Train service tends to be better on Sundays, but is often considerably less frequent than on weekdays.

Don't forget you must validate regionale train tickets before boarding.

Without car: stay in Florence, the capital of Tuscany (for Umbria it is probably best to stay in Perugia).
With car: stay in a small town or countryside that is central to the places you want to visit.

I know it's not on your mind, but consider Bologna as a base. It is well-connected by train to some great towns and it's small enough that you can stay centrally and have a 10-minute walk to the train station and a 10-minute walk to the heart of the old city center. Day trips include: Modena, Ferrara, Ravenna (gorgeous 6th c. mosaics), and of course Florence.

Thank you for your input which I will take into consideration. Between Rick's book and the wealth of information on the internet, I have to admit, it's hard to come up with an itinerary! I am beginning to realize that whatever route we take, (can't do it all in 2 weeks), it will be a wonderful experience that will draw us back to capture some of what we've missed.

Roberto has the right idea. Without a car Florence is best. You can easily train to Pisa, Lucca, Arrezo and bus to Siena from there and there are organized bus tours that go to various other spots like San Gimignano that are hard to get to easily on public transport. With a car base in a hill town at a hotel with parking so you can easily drive to various spots in the region. We have done this from a Hamlet near Siena for a week 30 years ago, from a place within walking distance of Lucignano about 15 years ago and for two weeks in Montepulciano three years ago. We loved doing this and would definitely recommend the Val d'Orcia area as a base as there are such lovely views, hill towns and abbeys in this region. For a few snapshots of our last trip: https://janettravels.wordpress.com/category/tuscany/

In Umbria we loved staying at St. Anthony's Guest House in Assissi; fabulous location and they have parking. But given the choice, we would pick the Val d'Orcia given how little time you have for this.

Florence itself with maybe a day trip to Siena easily occupies 6 nights/5 days though.

If you haven't already committed to unrefundable hotels for the trip, I would suggest finishing in Rome if that is where you fly home. Go to Germany or to Florence on arrival and do the other stops and then finish with your 4 nights in Rome. Having to rush back the day before the flight (absolutely necessary to be there the night before) essentially wastes a day of your trip and leave you harried at the end. We have done this and finally learned to finish where we fly home thus having a relaxing end of the trip that doesn't wipe out the benefits of having a vacation in the first place. The first day is jet lag day and miserable anyway, might as well use it getting to your furthest point. And the train from Rome to Florence if you want to start there is only 90 minutes and tickets can be purchased at the airport on arrival so you don't have to worry about late planes (more expensive that way but you don't risk missing the train and trashing the tickets)

If you decide to take a place in Tuscany, one idea might be to pick up the car at FCO and then drive around the ring to Tivoli and stay at the little hotel at the gates of Villa Adriana -- spend jet lag day wandering around the ruins, get dinner at the hotel, get a good night's sleep and then drive north to your base. We did this a long time ago and I noticed the hotel doesn't get very good reviews now -- but perhaps another hotel in Tivoli would work. You don't want a long drive after a flight and this would be a good easy stop and Villa Adriano is simply a wonderful ruin. The pool at the ruins was used as 'heaven' in the tv version of Angels in America.

Small towns in Umbria and Tuscany are tourist economies -- you won't get any 'small town culture' but a lot of very pretty tourist traps. We spent two weeks in Montepulciano and loved it -- it is so beautiful -- and it was a great base for touring but have no illusions about our authentic experience. Every local who lives in the historic center serves the tourist industry; it is all about selling to tourists.