There are two.
Empire State Building, it has one on the 86st floor (most popular, open air) and on 101 floor (not very popular, same views, more expensive, relatively small windows, closed in area). You see the whole city, on one hand you're looking at the entire Midtown and on the other you see Downtown far away.
Rockefeller Center "Top of the Rock". You're in the middle of midtown, surrounded by skyscrapers. Open air.
Sadly no observation deck in the downtown.
Empire State Building, it has one on the 86st floor (most popular, open air) and on 101 floor (not very popular, same views, more expensive, relatively small windows, closed in area). You see the whole city, on one hand you're looking at the entire Midtown and on the other you see Downtown far away.
Rockefeller Center "Top of the Rock". You're in the middle of midtown, surrounded by skyscrapers. Open air.
Sadly no observation deck in the downtown.