I would strongly recommend April. With Easter so late this year, the spring break crowds will be spread out so, while the parks will be crowded, they should not be anything close to the Christmas crowds. The mid-summer crowds may not be any better than April.
To me, the clincher is the weather. Mid April might be hot, but mid-July will absolutely be stiflingly hot and humid to the point that even sitting around a swimming pool (with water as refreshing as a warm bath) is not that comfortable. Unless you have the stamina for that kind of weather, you will enjoy yourself a lot more in April. Since you expressed a preference for a mid school year break, I would recommend that. Do a little planning and get to the parks early and you should have a wonderful time. We have done that several times because of having family members tied to school schedules, and March-April is my favorite time to visit WDW. If you think you will enjoy Epcot's Flower and Garden Festival, that is another reason to choose spring.