Assisi to Orvieto and Orvieto to Siene

You can actually travel from Assisi to Siena by bus and skip Orvieto all together unless its in your plan to visit and tour there. It is quite beautiful. To answer your question more directly, the train to Orvieto from Assisi is better as the runs are more frequent during the day. You will most likely change trains at Foligno. The train runs from Orvieto to Siena are also quite easy although you will change trains at Chiusi. Bus runs are not frequent.