It is completely dependent on what your interests are.
Get and read a guidebook.
I agree entirely with datthecoast.
The two cities are wildly different. The areas they are in are wildly different. People visit them for wildly different reasons. The kinds of people who live there are very different. The bread is different.
Do some research on your own, then come back to either this board or the Milan forum and ask specific questions.
Google (for what it's worth):
Milan, a metropolis in Italy's northern Lombardy region, is a global capital of fashion and design. Home to the national stock exchange, it’s a financial hub also known for its high-end restaurants and shops. The Gothic Duomo di Milano cathedral and the Santa Maria delle Grazie convent, housing Leonardo da Vinci’s mural “The Last Supper,” testify to centuries of art and culture.
Florence, capital of Italy’s Tuscany region, is home to many masterpieces of Renaissance art and architecture. One of its most iconic sights is the Duomo, a cathedral with a terracotta-tiled dome engineered by Brunelleschi and a bell tower by Giotto. The Galleria dell'Accademia displays Michelangelo’s “David” sculpture. The Uffizi Gallery exhibits Botticelli’s “The Birth of Venus” and da Vinci’s “Annunciation.”
Apples and oranges...
Florence is all about art and one of the big tourist destinations.
If it’s your first time to Italy, would probably go to Florence.