The main parks on whistler and blackcomb will still be good in april and may. Plus the World Ski and Snowboard Festival will be on mid-April and thats one hell of a good time.
In the summer, once the camps are finished on the glacier, they open the features up to the public, but they aren't maintained a whole lot and deteriorate.
I would go in the spring if I were you. The main parks are ridiculous so I'm sure you'll enjoy it. Plus, Whistler is pretty sleepy in the summer except for the mountain biking community.