O'Hana is a served to you, all you can eat, muliti course meal. We always enjoy it and the restaurant itself has games for the kids with a live musicisan, a fun, lively atmosphere and many of the tables have views of the castle and therefore Wishes.
Kona is in an unremarkable atmosphere (sort of a hotel coffee shop type thing but its stll nice), with no views of anything but the food is amazing. The dishes are much more creative and the desserts are amazing. Its just an order off the menu sort of thing with no entertainment. But again, the food is really wonderful.
My vote is both! Kona for lunch or dinner and dinner another time at O'Hana. They are both different enough and good enough to warrent two trips to the Poly in my book.