We like both for lunch.
Kona has more appetizers, and is good for hot meals and sandwiches, as well as incredible desserts. If you go here, try the stickey wings app and the Kileau torte for dessert (if you are a chocoalet lover). Kona will probably be less crowded at lunch, as it is a resort restaurant.
Plaza Restaurant has great sandwiches and salads. I guess they have burgers, too - I'm not sure as I don't eat them. I haven't had dessert here, but from what I have observed, they look great. If you don't want to leave the park for lunch, this is a good place to eat.
I guess I haven't helped much, as I really enjoy both restaurants. I guess if you are spending the day at the MK, you need to decide if you want to leave the park for the meal. I do belive that Kona is worth the trip, but Plaza is a fantastic option if you want to stay in the par4k!