We will be renting a car for 4 days while we are in Siena during the middle of September. My question is: Should be rent in Florence before we leave and drive to Siena or should be train to Siena and rent a car there?
We rented a car (autoeurope.com) as we were leaving Florence, and kept it for a week while we travelled to the various hill towns. We didn't stay in Siena itself, but at a B & B outside of Siena which had ample parking.
I'd suggest getting to Siena by either train or bus, then renting there. The bus is cheaper than the train, it runs very frequently, and it takes only 75 minutes. That way, you avoid the craziness of driving in Florence. We rented in Siena earlier this month, and it was a positive experience. Our stop after Siena was Florence, and we considered dropping off the car there, but the owner at our Siena hotel suggested leaving the car in Siena and doing the bus instead. I think it was a good suggestion.
Just bring along your own GPS with European map data, since the rental company usually can't guarantee one for you.
I think picking the auto up in Siena will be easier than fighting the traffic in Florence. I have driven in Florence years ago and it wasn't pleasant then. It has only gotten worse, especially with the no drive zones now.
Avoiding a car in Florence is a good idea. I did it some time ago and it was difficult at times. Smaller towns are much easier. Also, the GPS is a good suggestion - still have a good map but the GPS makes things a lot easier