You don't need tickets for the nighttime spectacular outside of your park tickets. I suggest watching Illuminations at Epcot. I personally think there's something really magical about watching Wishes! at Magic Kingdom, too.
Fantasmic! at WDW isn't going to be entirely new for you, but it does have some differences. There's a lot of Pocahontas stuff in it, and it doesn't have Kaa. Kaa is replaced by the snake form of Jafar. I've heard that the Fantasmic! at DL is the superior version, but they are different if you're interested in checking out the WDW one.