bus or train from Rome to Siena?

We have always taken the bus from Rome's Tiburtina Station because it drops you off right in downtown Siena; the train station is a couple of miles away all uphill. Additionally, you have to change trains in Chiusi but the bus is a 3.25 hour ride, straight through. Earlier this month, the bus was 23 euros per person. There are also advance purchase fares which are cheaper but you're locked into that bus and I prefer the ability to change if necessary. I think the train is under 10 euros so it's a matter of convenience.