If only 2 nights you must take into consideration ease of getting to the destination. You can fly non stop into Paris from most cities so that could be best choice for 2 days. I love feltrinelli but think it deserves 3 nights and not that close to a major airport. Also agree is better later in the year so you can enjoy the lake.
FS Florence is also wonderful if you can fly directly to Florence for the two days. It is a gorgeous hotel and in a lovely park with a great pool and spa so you feel like you are in a resort, but just a walk to the heart of the city. If you cannot fly directly into Florence I would go to Paris to minimize travel time.