Orvieto and Siena, or spend time just in Siena?

Both are wonderful. I'm usually not a fan of one night stays, but with 4 nights I would do Siena 3 nights and Orvieto 1 night. Siena is bigger, has more sights to see, and is prettier to just walk around. I stayed in Siena for 4 nights and Orvieto for 2 nights. With 4 nights, I would do Siena 3 and Orvieto 1.

I haven't been to Orvieto, but plan to do so. I have been to Siena, spent two nights there and that was enough. I would split time between the cities.

I would split the time between the two. We have stayed in Siena a few times, and I think 2-3 days would be the max unless you’re planning day trips out to surrounding Tuscany towns or wine tours.

On our last trip we spent two nights in Orvieto and two in Siena and enjoyed both. Orvieto is more quiet but after busy Rome it was a lovely respite. I was glad to have a full day there to enjoy the town.

I've stayed in both and would do 2 nights each.

Thank you all for your responses. It is very helpful. We are leaning toward doing 2 nights in each town, but not sure yet : )