I have not done this itinerary on the Crown, but have done it on the Norwegian Dream a couple of years ago. The ship is secondary to the itinerary and you will love it. Nancy Norris wrote a MEGA review of the cruise located at:
December is an excellent time to go and you will experience hot weather that goes to cold and then hot again. I would easily spend a couple extra days in Buenos Aires, as it is a wonderful city.
There was a group of us on the cruise and we did our won thing in all of the ports, which worked out pretty well. Be sure to make your way to Punta Tombo penguin reserve, as this is the major attraction on this cruise and well worth the hassle to get there.
Have a great cruise!