My history: 5 trips to Venice - 3 trips overnight and up to a week - have stayed in three neighbourhoods in Venice proper and also have done 2 day trips to Venice.
Florence 9 trips - most for at least two weeks, others for one month
I'm a Florence person and would pick it over Venice any day.
But, my interests are in Renaissance art and history and I would say that for some people Florence is a city best appreciated once you get to know it. For a person who is not really into museums or art, the attractions of Florence might not be immediately apparent. But it is a beautiful city and in my mind well worth the time. Whre else can you walk the streets where, during a short period of history, you might have passed Michelangelo, Leonarda Da Vinci and Machiavelli? Not to mention the other periods of time where Florence was home to many other seminal figures like Dante and Gailleo. And the sight of Michelangelo's David in the Accademia was a life changing experience for me with regard to art. It is NOT like Rome. Nor is it the Tuscan countryside if that's what you want to see.
Frankly, if I never had to go to Venice again, I would be very happy. But I believe everyone should see it once, so I'm going once again in March of 2014 because the person I'm travelling with has never been. For me, Venice is more of an "eye candy" type of place and novel because of the setting. But I don't feel one experiences it as a living city, more like a tourist attraction. I personally have enough of Venice by doing a solid one day visit there.
If you have some very specific sights you want to see, I would recommend that you add them up and pick the place that has the most of importance for you.
And, consider that at some point you may go back and then you can plan to see more of the places on your list.