I would agree with Chris Wu that you really should make Florence more than a day-trip; it's one of the most incredible cities in the world. And I wouldn't stay at the Sheraton Firenze; way too far out to be able to enjoy the city's ambience.

Hotels in both places are very expensive, and the weak dollar against the Euro makes it worse. Still, you're not going to enjoy or get a sense of either place if you're staying far out. If you don't have enough *pts, or can't get awards at the SPG properties in both, I would use the pts. you do have to be at a well-located SPG property in one of them. For the other, I'd search other hotels or perhaps rent an apartment? Many properties in both cities now do 3-day minimum rentals. Or, save Rome for a later trip, and combine Florence with a less-expensive city, like Siena or Bologna?

It's hard to have a bad time anywhere in Italy, and you want a reason to go back!