Both cities are great. It all depends what your tastes and expectations are.
You shall visit these cities during high season. Both will be very crowded. Depending on the weather, both cities will be equally hot, Venice might be quite smelly. At both places it is going to be expensive. Terraces in Venice have prices close to highway robbery.

If you go for sights (buildings, romantic, water, bridges) Venice is the place you should stay longer. Maybe take a sisetrip to the blowing glass factories at Murano.

If you go for culture, museums, nice buildings, good food, nice terraces to enjoy life, more affordability, choose Florence as the place.

As I don't know what you are looking for, I can say no more at present.

Enjoy both anyway, both worth to see, but Florence is my favourite, especially because you can at least get away from the tourists if you try. Not possible in Venice.