Well, I think both of the restaurants have GREAT food. But if it's atmosphere you are going for, 'Ohana wins in my book. Kona is an open cafe, and you have a view of people coming and going inside the Poly, but no outdoor view (I guess you might be able to see the monorail if you are seated on the far side of the restaurant- I'm not sure cause we were seated on the other side both times). There is also no entertainment at Kona. I had the prime rib one night and the veggie spring rolls another night and both were EXCELLENT. The service was also very, VERY fast and friendly both nights we ate there (they we not busy either night).
'Ohana- what can I say except it was our favorite meal on our 10 day trip last Feb/March. And we ate at a LOT of Disney restaurants that trip. The service was great, we loved the food, and the atmosphere was very relaxing. We were lucky enough to get a PRIME window seat facing the castle and they shot of the fireworks about 20-30 minutes into our meal. We were there with my parents and my MIL and both were just amazed at the food and view. We all wanted to cry with happiness when we were seated! (We had had a view bummer meals before this meal so this sure made up for those disappointments).
I loved both restaurants- in fact, they both easily make my top 5 faves in WDW, maybe even top 3. But 'Ohana wins the atmosphere mark in my book.