My family loves both. If you are big meat eaters, go for O'Hana. The food is all you can eat, and the meat is the highlight. There are also some activities for the kids, and you get a great view of Wishes and the Electrical Water Pageant if you are there at the right time. The Mai Tai is to die for.
Kona Cafe is one of our favorites. The asian inspired menu is great. If you want seafood, they have some nice offerings. Some of the appetizers are enough for a meal themselves. Especially the chicken wings. Yummo. The desserts are exceptional there, far better than O'Hana. The Kiluea Tort is great. There is also a macademia nut tart kinda thing that is pure heaven.
You have a very tough decision. If you can, try to work both places into your schedule. Both are reasonably priced and will give you a nice meal. Sorry if I'm no help. I love them both.