Summer. Here's why:
1. Spring Break, if your's falls right in line with many other schools', is extremely busy. Often, it's busier than summer. Mid-July-Early August midweek will probably have lower crowds than Spring Break.
2. As PP mentioned, this spring the Rivers of America will be drained = no Fantasmic, no boats, no rafts to the island, and lots of construction walls.
3. World of Color, new nighttime spectacular at DCA, will be opening in late April. I wouldn't miss this new show, so summer would be a guaranteed time that it's running. Nobody is quite sure exactly when it's supposed to premiere, so it's risky during April.
4. Full entertainment in June, including (hopefully) Disney's Electrical Parade, Magical! fireworks show, Fantasmic, etc. Fireworks would be the only one of the three shown during Spring Break (although I agree with PP in that it is a better show).