The heat is about the same---but I would plan for July/August over a trip that is in a week. You will be more likely to get better fast passes and even dining reservations. That said, if you aren't planning on eating at some of the harder to get places like the character dining at Artist Point or dinner at Ohana and don't want to ride some of the harder to get rides like Slinky Dog then you might be fine doing a last minute trip in June. I would check rates for hotels and see what prices out better.