I've never been to the Luau. But we just saw the HHD three weeks ago. It was fun. The food was pretty good by "Disney" standards. It was one of the better meals we had in our week at WDW. I'm sure they offer a vegetarian substitute. But they did have corn, bread, salad, and strawberry shortcake. (BBQ Ribs and Chicken too.)
Overall, I'd go again. And I'm usually more for of the "been there, done that" type of person.