Best way to get from rome to Siena?

The bus is most direct and efficient and has more schedule options. It also drops you off near the center of town and within walking distance of central hotels. The train station is outside town at the bottom of the hill - you either have to take a cab or bus to get to the city center.

Ditto Douglas. The bus station for the bus to Siena is not the one immediately outside the main Florence train station. It's actually about 1,000' feet South from the train station.

I took the train from Rome to Siena last year, and that option also works well. You'll change to a local train or bus in Chiusi (bus departs from in front of the station, to the left as you exit) and will need to walk across the street to catch a city bus into Siena center, but it's all pretty easy. Any city bus marked Via Tasso stops near Piazza Gramsci, so no need to wait for one marked as going to the piazza.

Just FYI, there's an escalator from the train station to the top of the hill. It's been there at least a couple of years and must be one of Siena's best kept secrets. As you exit the train station, walk across the street to the shopping center and there it is. It dumps you just outside the walls. We used the train station a couple of times for parking and found it very convenient even when carrying groceries or luggage to our apartment in the center. We were about the only non-university student types on it so they must not advertise it to the tourists. Enjoy your trip!

Thanks for all the great suggestions!