Rome isn't too bad for traffic. Can get a bit hectic alongside the Forum.
The only place that I would not cross is over to the gardens in the Piazza Venezia. I can see now why there was never anyone in there.
Florence is mainly pedestrianised and the only traffic of note is around the railway station.
The only place in Venice where you'll find a car is the car park by the bridges from the mainland.
I did see a car in the centre once though when they were filming a commercial and as we entered what we thought would be a deserted square we came across a car suspended in the air on stilts.
Came as a bit of a shock.
Go with Rome if your partner wants that but myself I'd say Venice. Simply the most stunning place in the world.
Start your love of Italy with a bang and go there.