I would say spring break, with one caveat: different school systems have different break times. That makes them slightly less busy in WDW, but also, if you have people traveling from different states, it might not work. College breaks are typically second week of March. In the Northeast, it's typically a week in February, week in April. A lot of places do Easter week (end of March this year, mid-April next). Here in NC, do the week before Easter. Go figure.
So, if you all have the same break schedule, I'd go then.