As a general rule, almost all school districts take their Spring Break either the week before or the week after Easter. If Easter is early, all of March will suck but April will be fine. If Easter is later, the reverse will be true.
Since Easter will fall on April 20 in 2014 it will skew the times later than normal. It will probably be pretty reasonable through March and the first week of April, while mid to late April will have heavy Spring Break crowds.
Early March or late April is probably the best. Smack in the middle of that time frame is the Spring Break crowds so it will be crowded. I went the second week of March last year and had a great week. Low crowds, nice weather, and the extended hours had already started. If I would have gone the following week it would have been during the Spring Break crunch.
Please... put Guardians of the Galaxy in Tomorrowland.
With Easter being on April 20 next year, late April will still be Spring Break. The whole Spring break season will be later than usual due to Easter being later than usual. Although they call it "Spring Break" now and not "Easter Break" it almost always begins or ends with Easter weekend.