There really is not a bad choice, IMO. If you're goal is eating great food then choose Kona - if you are interested in eating good food in a fun atmosphere then choose Ohana.
If you're looking for a traditional sit-down, order off the menu meal then I would suggest Kona for breakfast or dinner. There is a much broader selection of foods at Kona since you're ordering off the menu & the food is better than Ohana.
If you're looking for a fun yet relaxed character breakfast than Ohana would be a great selection. Pluto & Stitch are a riot & there is a dance for kids with the characters around the restaurant. Also this is one of the few character breakfasts which is not a buffet - which is nice so you don't worry about missing characters. The food is good - nothing special: mickey waffles, eggs, etc.
If you're looking for a fun dinner with a strong Hawaiian vibe, then I would suggest Ohana. Again the food is good, nothing great: appetizers like chicken wings are served family style. Various meats (pork, steak, shrimp, turkey) are served from skewers. There is usually some constant form of entertainment going on like learning to hula dance or doing the limbo. Some people find Ohana noisy rather than fun.