Well, we've been to DL in March during spring break 2 years ago and it was CROWDED! Wall to wall people and long lines for everything. We'll never go during that time again - just waaaaay too many people.
It seems to me that a large majority of schools have spring break in March, whereas a smaller number of schools have spring break in April. I'd go in April if I had the choice.
When we lived in Arizona, most (if not all schools) had spring break in March. Now we live in Washington State, and our son's school has spring break in April. It all depends on when the school year started. If school starts in August, spring break is usually in March. If school started in September, spring break is usually in April.