Hi Jan,
I visited New York over the Xmas holiday in 2009 so can offer some advice regarding tickets for Top of the Rock and Statue of Liberty/Ellis Island.
I bought tickets on the day for Top of the Rock with no problems at all. You can buy a day ticket, a night ticket or a combined day/night ticket if I remember correctly.
Regarding the Statue of Liberty/Ellis Island I would recommend that you buy tickets in advance of your trip as the queues for people buying on the day can be very long. In addition, if you want to get up as far as the crown you will definitely need to book in advance as there is only room for about 12 people at any one time, and therefore only a small number of tickets are sold for access to the crown each day.
Top of the Rock is brilliant; on a clear day you get great views looking south towards the Empire State Building and onwards to the Statue of Liberty, whilst looking north you get great views over Central Park.
Statue of Liberty is wonderful whilst Ellis Island sums up the melting pot that is New York, telling as it does the story of how immigrants first arrived in the city many years ago. It's well worth stopping at Ellis Island on your way back to Manhattan; just leave enough time (maybe a couple of hours) to do it justice before you get the ferry back to Manhattan.
Hope this helps and have a great time.
Cheers
Mike