I voted for the "nighttime" choice because of the following reasons:
1) I think it adds to the spookiness of the ride
2) I think it's really cool to ride it at nighttime (you get to see the park all lit up)
3) Your not blinded by the beautiful sun like alot of other people said
Please, please don't get me wrong, I love to ride the Tower of Terror during the day, but riding it at nighttime has always been a dream of mine.