Another vote for May. For starters, March is the start of Spring Break season so the number of people in the parks (and as a result, the rates) are higher from all the price checking I've ever done. May is sort of a down time between the Spring Break rush and the start of the summer rush. Plus, in May the weather is usually more in line with what you think of as mild Florida weather (usually rather mild in the upper 70s and 80s; occasionally in the lower 90s) while in March it can sometimes still get a little chilly, which is fine if you're not planning on swimming.