Having been to Milan twice and Florence 3 times I can say that both cities are lovely. But as cla114 states, Florence is that little more magical and more aesthetically beautiful. You can indulge in the many cultural/historical features through galleries and museums, you can avoid them all and just wander through the streets and admire the views or you can mix both aspects on your stay in Florence.
From a logistical point of view, the trip is very straightforward with a 1.5 hour train journey between Rome and Florence and a 2.5 hour train journey between Florence and Venice. If you are able to fly into Rome and out of Venice (or vice versa), this will minimise a return journey between Venice and Rome.
Cheers
Mutiny