I prefer the food at Kona. I'm not crazy about the atmosphere. It's a big open space and kinda like you're just sitting off the side of the lobby. But the food's amazing.
Ohana is a lot of fun. Great atmosphere. It can be kinda loud with all the kids activities going on. Excellent view if you're sitting by the window. Tons of good food. You'll walk out feeling bloated and ready for a nap. It's a unique experience, and you should really do it at least once.
If you're choosing a restaurant based only on food quality, I'd pick Kona. But you can have a great meal anywhere. Ohana's offers a unique experience you probably won't find back home. So while you're on vacation, I'd recommend Ohana.