Normally, the earlier you go in March, the better. But in 2019, keep in mind that March 5th is Mardi Gras. That tends to bring in some big (and sometimes rowdy) crowds to Disney. But it's manageable! I agree that Flower and Garden is fun at EPCOT. Something else to consider - if you're into baseball. The Braves play spring training games at WWoS - their spring season will have started by the time you're down. Have a great trip!