I am thinking if you have a car, you really should stay in Costa Mesa or Huntington Beach. The hotels near the park, in the summer, are going to be fairly pricey.
There are various hotels in Costa Mesa and inland Huntington Beach that would cost about the same if not less during these same months. The huge advantage to this is that you are near the beach (not on the beach unless you want to pay a LOT of money) and it is a LOT cooler. You also will not have to deal with a million tourists crawling all over the place, probably cranky after a long, hot day at the park.
It seems you are already familiar with the parks, and if you know them well enough you can do both of them in about two days, leaving the other days to come and go as you please, hit the beach, go to Hollywood or do whatever suits your fancy.