For you, that should be what your interests are. Are you into roman history, or interested in seeing the treasures of the Vatican museum and the largest church in the world, then go to Rome. If you like medieval and renaissance things, or have interest in the foods and wines of Tuscany, go to Florence.
Me, I tend to be more budget minded in my travels. I would base my trip on whichever city was cheaper to fly in to, and work my itinerary around that. My next trip to Italy has me flying off season onto Rome and out of Milan, because that was the best deal I could get. If all things were the same cost, I would base the flights on whichever city had the best flight itinerary or took the least amount of time to get to where I want to be. Flying from the west coast like I do, sometimes those cheaper flights could have me stuck for obnoxious amounts of time, such as having to do an overnight in Seattle airport like I will be doing on my way home.
If it's really MY opinion you are interested in, for my upcoming trip I will be staying 1 night in Rome before I head to Sorrento, but I will be spending a total of 8 nights in Florence & the Tuscany/Umbria area. So my vote is Florence.