Hi Nicole,
According to the crowd calendar on the unofficial website Touring Plans, there isn't much difference at all between your two proposed sets of dates.
For 27 November - 5 December 2012 the crowd level varies between 1 out of 10 to 6 out of 10 with an average of 2.9 out of 10 for your entire stay.
For 4 - 12 December 2012 the crowd level varies from 1 out of 10 to 6 out of 10 with an average of 2.4 out of 10 for your entire stay.
I think that if it were me, I would go with 4-12 December 2012 as you will be able to see more of the gorgeous holiday decorations and offerings.
Another reason to choose this timeframe is because the following parts of the New Fantasyland will be opening in time for the Holidays 2012:
Under the Sea ~ Journey of The Little Mermaid, Enchanted Tales with Belle, Be Our Guest Restaurant, Gaston’s Tavern, and Ariel’s Grotto. The later that you leave your trip, the more likely that you will be able to experience them all during your visit.
Wishing you a magical trip filled with memories which will last a lifetime!