That's definitely a hard one... Crowds are going to be heavy either way so for the sake of the heat, I would probably say March (although as the PP said, it's probably going to be hot either way). I would definitely take advantage of FP+ wherever possible and get an early start in the parks. Part of making a trip work during busy times like that is to not go in there expecting to see EVERYTHING in the park and that there's going to be a lot of people.
Too many times I've read of people going during spring break, staying in the park all day and then complaining that it was crowded... It's spring break. It's going to be crowded.
If the heat is your biggest concern, I would get an early start in the morning, stay in the park until 11 am, go back to the hotel and relax by the pool, then go back to the parks around 4 or 5pm. You'll miss the heat of the day and depending on the park hours, you can still get six or seven hours in a park.