Flight
Flights into Rome will likely be cheaper as it is a larger airport of the two. It also provides more options when taking the train to SIena.
Flight
Florence is closer to the venue but has a smaller airport.
Train
The easiest way to get around is by renting a car at whichever airport you land at. Otherwise: From either Rome or Florence, take the train into Siena. In Siena, you can either take a taxi or rent a car to drive to Rocca D'Orcia (offers restaurants of every sorts) or Castiglione D’Orcia (offers amenities for everyday needs).
Hotel
We've put together a list of recommended accommodations, ranging across different budgets. It is recommended all guests stay within 10 minutes of the venue (address: Località podere Vignalunga 2 Castiglione d Orcia 53023 Siena)