Jaime,
If I were choosing between the two, I'd go for May. March and April tend to be busier in the parks, since it seems it's always spring break somewhere. In May, most of the spring breaks crowds are gone, and the summer break crowds haven't yet arrived. We've always found this to be a great month to visit.
Also, if you go early in May, you will still be able to enjoy the Flower & Garden Festival in Epcot, which is really spectacular. Even if you aren't especially into gardening, children of all ages are amazed by the character topiaries throughout the park at that time. See some examples of the artistry
here.
One last thought - don't overlook the water parks. When my daughters were younger I didn't go because I thought there wouldn't be enough for them to do, and I regret that now. In true Disney style, the water parks are amazing and have fantastic, over the top areas for even the littlest of children.
Have a great trip!