After visiting both the observatory at the Empire State Building and Top of the Rock they are both awesome experiences. But if you only have time for one.......I would personally choose Top of The Rock. Why??? My simple answer is this - When you are on top of the Empire State Building - one of the cities most beautiful and iconic buildings is missing from your view.....because you are standing on top of it! Also the observation areas at the Empire State Building are small which makes them quite crowded, and at least when we visited the lines were a lot longer and since they can not accommodate as many people they move slower as well. That being said - if you are near the Empire State building I would certainly stop in the lobby as it is absolutely stunning!!!!! You will be glad you did! But I would hit Top of The Rocks observation deck.
We have visited in the day time and at night and again both are different experiences but both are amazing!
Rockefeller Center is such a fun place to visit! Some really neat shopping areas, inside there are shops and restaurants, the NBC studio tour, Jacques Torres chocolates (one of my favorites!), the Channel Gardens are beautiful and its just a neat vibe to walk around there.
I'd love to visit NYC in the Winter someday......and when I do I will ice skate at Rockefeller Center!!!! =)
For more information on Top of The Rock visit http://www.topoftherocknyc.com