The crowds in March can be very large due to spring break. We were there 3 years ago that time and it was awful. There were hour long waits on a lot of the rides. The only difference between that time and June is that it probably won't rain as much and be as hot.
TouringPlans.com lists that week in March as "7's" and "8's" while June is almost all "8's". Not really a big difference, but at least you can say it isn't more crowded in March.
If you go in March, try to go before the 7th, say the 1 st to the 7th. Actually, the VERY best time to go is next week. We went the week after winter break and the parks were like a ghost town, compared to the usual crowds.
Most all of Florida's school systems have their Spring Break 3rd or 4th week of March so you get a lot of locals during that time in addition to out-of-towners on Spring Break.