Hi Rebecca,
During spring break times, like March, it's difficult to predict which parks will be less crowded. They will all be fairly busy. You can still visit lots of attractions and have a fantastic time in a busy park.
We have late sleeping teenagers now, so we typically avoid parks with morning Extra Magic Hours. I will mention that I looked at March 21, the date you listed for your trip, and that date has morning Extra Magic Hours listed for
Disney's Hollywood Studios and evening Extra Magic Hours (until 3 a.m.!!!) for the
Magic Kingdom Park.
If I can help you further with this question, or answer any other questions, please let me know!
Wishing you a magical vacation,
Amy H