3 nights to spend in either Sienna or Florence. We are not the biggest art buffs but would plan to see the major sights in in flornce. But should florence only be used as a day trip, while basing ourselves in sienna?

Honestly, they are not that far from each other, so I would do 1 night in Sienna and 2 nights in Florence. But since you are not art buffs, maybe the opposite? Either way, I would try to see both.

What are some of your considerations? For just one example, if you think you'd prefer staying in a smaller city with far less traffic in the central core, then I'd recommend Siena. If not, then I'd recommend Florence.

One thing to consider about Florence, the art museums there are closed on Mondays. You said that you aren't big fans of art, but most people would not pass up an opportunity to see Michangelo's David. As for where to stay, Florence is a much bigger, bustling city than Sienna. You don't say if you'll be driving, but know ahead of time that Sienna is virtually carless. I'd see what hotels might be available in each. Perhaps you'd be swayed by that. Enjoy your trip.

personally i would stay in florence and make siena a day trip. i stayed in florence one night, two full days and could have stayed longer. theres so much more to do there. i limited myself to two museums, the accademia for the david, and the uffizi. and felt i saw plenty of paintings of mary and jesus but was worth it for the few i really enjoyed.
siena felt significantly smaller. and very manageble in a day. i took two day trips there and only spent 3 hrs there the second time.
mostly it depends on what your likes are. florence for big city always something to do, siena for quieter slower days. and the shopping in florence is far superior to siena (something i always enjoy)

Please be aware that there was major roadwork going on back in June, that forces you to take a detour coming and going between Florence and Sienna. It will add some time to your day trips. That said, if you want more of a nightlife, stay in Florence. If you're looking for a more laid back place, stay in Sienna.