My vote would be for US/IOA. I like Seaworld but it really hasn't changed in quite a few years. They have added maybe a couple things in the past few years. There is alot more to see at US/IOA. It depends on your preference. This is just my opinion.
OK, so I know this is the Universal Discussion Board but with a six year old, I would say SeaWorld. But it may depend if the kid is an animal lover or a thrill seeker. There may be less from an adult point of view but there's probably plenty to do for a kid's point of view.