The towers are tricky as the view is impressive at various times and changes so much with the light. Early morning can be fabulous especially if the haze is still lifting with the added bonus of no queues as Tourists don't like getting up early, obviously sunset is great if the conditions are right and then you really need to go up after dark for the New York skyline by night. Everyone says and they are generally right go up the Rock for the views as you then get to see the Empire State rather than being up the empire state and not having it in your photos. Lot's depends on if you are there for photography or to see and experience lots? If there purely for the photography then book a couple of trips up the rock if not I'd go for Sunset like you've planned as it will give you the great view, great photos and if you hang around an look at nyc by night.
Most importantly enjoy the trip NYC is one of my favourite places and one of very few to make it onto my must actually go back list and not onto me just talk about how nice it would be to go back list