Seeing as it's your first trip tp WDW, I would recommend sticking to Disney rather than skipping something at WDW to go to another theme park. The 4 Disney parks are so different from each other that, I find, you are able to go day after day without getting the feeling of being Disneyed to death.
If you do have time and the money (it's true the tickets are not cheap) then I would recommend Seaworld. You can see the whole park in one day, but they also usually offer free entrance for the next day, which is good if you happened to miss a show or think you might want to go back , even for a couple of hours, to do something you particularly enjoyed. Our favorites are the Shamu show and the dolphin feeding. Also, there is a huge play area behind Shamu stadium which will keep the kids busy for an unlimited time.