Heat or crowds?
The spring-time is certainly nicer weather-wise, as by June, it's almost always VERY warm - even in the evenings - however, you might have more crowds at the time of spring break, so it really comes down to personal preference. If you like it HOT, then June would be fine, otherwise deal with the possible crowds and go in the spring. (P.S. - It can be quite crowed in June too, and then you have both negatives, so I'd personally choose spring). Of course, you should have a great time whenever you go.