Hi Sarah,
We love both Disneyland and Walt Disney World and each have their own special magic!
We have only been to Disneyland in August and neither time did we find the park incredibly busy. We did find it interesting that Disneyland has one set of guests in the morning and a completely different set in the evening.
As for Walt Disney World in March, you will have students on spring break throughout the month. Also, you need to look when Easter is as that week will be busier than others. Otherwise, March is one of the slower periods at Walt Disney World to enjoy.
One difference between Disneyland and Walt Disney World in the number of
FastPass+ available. You will want to schedule them in Walt Disney World as it significantly reduces your wait.
Wishing you a magical vacation!
Beth