Looks like we can catch either a 1230 train or Sena bus to Siena with a morning arrival at FCO. According to the schedules, the bus arrives about 1 hour earlier than the train and in the center of Siena. Any compelling reason not to take the bus?
I suggest the bus. Non-stop and pretty comfy.
We took the bus as well. You'll have to transfer at Chuisi if you take the train, the bus was direct.
I take the bus every time I travel between Rome and Siena for the reasons listed above, but also because the bus drops you off in town rather so that you don't have to get a bus from the train station into town. The bus is easy!
Ditto the bus, but we did find that it was just a bit less complicated, on our departure from Siena for Milano and the return flight, to taxi from our hotel (Palazzo Ravizza) to the train station at the bottom of the hill to be able to clearly indentify the departure bus stop for the Sena bus to Milano. At least as explained to us by the hotel, some of the interim, i.e., bus stops other than the train station, are fairly informal and may be hard to identify. We took the hotel's advaice and the bus departure from the train station's bus stop was quite straightforward and it seems like only yesterday (in fact, it was just the day before yesterday).