We have been to WDW the last week of April (22-30) twice now and LOVED it both times! The weather was fantastic (not a drop of rain) and the crowds/lines were light. The parks close a little earlier that time of year but we took advantage of the extra magic hours in the parks and were able to do and see everything that we wanted.
March is Canadian spring break time and the park is VERY crowded that week. This year, it is the week of March 12-16. If you decide on March, avoid this week at all costs!
We go the first week of March, the weather is nice some days nice enough for swimming some we needed a light jacket or sweatshirt. But the crowds were not bad at all! That is the only week in March I believe that it will not be packed.