I agree as well, but if the OP's only got one day, I would pick Milan over Florence specifically for that reason - IMHO, there's too much to see in Florence to spend just one day there. Too many museum choices, the Duomo, the markets... I suppose you could choose one museum and see the Duomo late in the day, but I personally cannot imagine cramming all of Florence into one day. In one day in Milan you can visit the Last Supper (as long as you book in advance) and the Duomo and browse the shops. But to try to see Florence in one day... my head would explode!