We've done both times. I would say it all depends on when your April break is. If it is either the week directly before or after Easter, I would say President's week is better, if you can move a week away from Easter, then the crowds become much more manageable (although still crowded).
Both times are going to be crowded, as many of us know (my DW is a teacher, so those are the times we can go too). But if you just don't go in planning on doing everything in one day, and remember that you're at WDW, you'll have a wonderful time.