Really kind of guessing here (as you are!)...but by mid-May on, more and more colleges are letting out...and crowds slowly start to build. Easter is so late this year that you won't have to worry about Easter-break crowds last weekend of March.
Honestly, you're probably okay either weekend. We are big swimmers in our family (we're from New York) and we want swimming weather (day and night) if we're going to spend money to go to Florida. Factoring that in, we'd probably pick late May...but if swimming is not that big a deal to you, I'd probably lean toward March!
Hope that helps. And you know what the good news is.....no matter which you pick, you'll have a great time!!!
