We went in March and June this year. The weather was nicer in March (cooler--although it was too cool to swim in the evenings if that is a consideration--we could swim in the afternoons though). March was busy because of spring break.
June was hot and crowded, but the park hours were longer, the daylight lastest longer, and we could use the pool for more hours.
If I had non-school age children, I would go in February. I have heard that is a nice time to go, or in September right after kids go back to school.
Of your two options, I would pick March if you want cooler temperatures in the park, or June if you want to have longer hours in the park as well as more time to use the pool.
DJ