Go here and read some of Rick's articles on Rome & Florence, especially the "Plan your time" ones.
http://www.ricksteves.com/plan/destinations/italy/italy_menu.htm
It depends on what you're more interested in, but generally I would weigh more time towards Rome. If you are leaving 12/22, you will land on 12/23, and 1/2 will also be a travel day, so you'll lose one day on either end of the trip. By my count that gives you a total of 10 complete days in Italy.
If I were going, I would probably visit Rome for 4-5 days, stop over in Orvieto for 1-2 days, and finish up the rest of the time in Florence.
Reservations--you will probably want reservations for the Accademia and Uffizi in Florence, and they're mandatory for the Borghese Gallery in Rome. Not needed for the Colosseum--instead look into getting a Roma Pass (often discussed on this site--see the top of the board for a thread on how to search effectively). You may want to pre-book tickets for the Vatican Museum, but if you have a flexible schedule, you probably will be OK without. Just don't go near the place on Sunday, as it is the last Sunday of the month and the museum will be free & completely packed.