Not been to NYC. Been to the falls twice, summertime and April. The April trip was not great the snow was dirty, melting and it made everything seem so dingy. Summer was a good time.
The city has a bunch of different things to do in all seasons. If your an outdoorsy person then definitely come in the spring or summer and hang out in central park. During the winter you could go to one of the million different museums and restaurants that the city has to offer