I would suspect the train would be the better way to travel, unless you are taking a bus that does not stop between the towns. The roads are good, but that could end up being an all day bus ride both ways. Italians tend not to travel by bus, but do travel by train.

The drive, by car, from Siena to airport will be at best one and one-half hours, so would be okay if you will be leaving in the afternoon. If you are again planning public transportation, I suggest being closer to the airport no matter what time your flight will leave.

Have a great trip! Lisa in Chicago