Hi there, Tracy!
Thanks so much for checking in with me!
I would suggest the earlier March date. You will find many fewer Spring Breakers in
early March as you will in mid-late April (the local university, for example, will take its break in mid-March). That's true for universities and "younger student" schools.
While attendance at the
Walt Disney World Resort does see an uptick starting with Presidents Day weekend and pushing towards summer vacation, the real "peak week" of Spring is Easter Week, which, in 2012, falls on April 1 - April 8. The week thereafter is also higher than average in attendance. The closer you can come to February - and away from April - the better off I think you'll be in terms of crowd levels.
Sites like TouringPlans.com do collect historical data on attendance, and demonstrate a high level of reliability in their predictions of upcoming dates based on those figures. You could certainly check out the sit and see what those statisticians anticipate crowd levels being in your two best weeks.
One more reason I am championing March: The weather!
The weather in March is pleasant, with highs in the mid-upper 80s and lows in the mid-60s. The rainy season has not yet begun in earnest, and you will definitely get to make use of the pools at the Resorts. You can wear shorts during the day, jeans at night, and you might even have an excuse to purchase that adorable hoodie you've been eyeing in the shops.
Have a wonderful time, and please let me know what dates you ultimately chose as your travel time.
See you later, celebrator,
-Deborah B.