Both date ranges are going to be extremely busy. Memorial Day weekend will probably be less busy of the 2 choices though. Spring break will be "insane" crowd levels. Memorial Day weekend will be Heavy to Insane.
With a good plan though even the busiest days are manageable though.
Not only is your March 30-April 3 your spring break, it's also Easter (April 1) weekend...very crowded. Memorial Day weekend is crowded too, but not that crowded.
Since it's Spring Break for you and likely others as well on ANY of those dates, I'd say you'll be sharing the Parks with tens of thousands of other guests. It's a coin toss and "least crowded" is probably a statistic that involves a difference of like, 32 guests. Spring is just NOT a time of year that is ever described as "least crowded".