Never had dinner at either restaurant. Last trip we did lunch at GFC and a breakfast at Kona Cafe. Kona is on the 2nd floor of Poly, above the lobby, which makes it feel less like a distinct, separate eatery. But the food was good, no doubt about it.
Grand Floridian Cafe is a separate room, and feels somewhat precious. The food was fine.
I was just checking out both restaurants' dinner menu. To be honest, neither one is making my mouth water, but hey, I'm picky. I'd suggest, if you haven't already, to check out the menus, too. There may be one place with entrees that interest you more than others. With both being on the monorail, getting to either one shouldn't be too difficult or time-consuming. I really do think it would boil down what has a more appetizing menu to you.