I'd recommend reading the guidebooks, as it is all a matter of personal taste. For me personally, Paris was my favorite place in the whole world, whereas the 1.5 days I spent in Florence felt like enough for me, and I didn't particularly care for it. On the other hand, one of my good friends says Florence was her favorite place in all of Italy. So in the end, it is a matter of researching what's in both cities and seeing what you are most interested in. It also depends on whether you are going to try to use Florence as a base for doing day trips into Tuscany, in which case you'd need extra days there. Overall, if it is strictly Paris and Florence that you're visiting, and you are equally interested in both places, the deciding factor would probably be size - Paris is a much larger city with much more to see. You can see the main museum in Florence in a couple of hours, but the Louvre in Paris could easily take you all day (or many days, if you wanted), so you'll need more time in Paris.