The first deck is on the 86th floor and is outdoors. The second one is on the 102nd floor and is in a small room with windows all around. The second has a better view and fewer people (since visiting it requires a separate ticket), but also very little space to move around. On both decks, you walk around the edges looking at the city.
There is no big difference of the view of the city. The first deck is high enough to see all NYC. In my opinion the second deck is optional. For shorter wait you have to pay VIP ticket, you can pass everybody who is waiting in the long line before security check.
16 floor and fewer people
One is taller than the other
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