Kona has Tonga Toast while GFC has Lobster Benedict. Actually each has items the other does not have. I would suggest you click on <Dining> on the blue bar at the top of the page and then go to menus.
I agree with Cheshire Figment above, take a look at the online menus. I think allears.net has the most up to date but don't hold me to that!
I've eaten at both and really like them both too. Grand Floridian Cafe is pretty standard breakfast food like eggs, waffles, etc. Kona has more interesting dishes for breakfast like the tonga toast, macadamia pancakes and somoan egg with pork hash. I suppose I'd give Kona a slight edge since I like the more interesting menu there. Honestly, I don't think you can go wrong with either!
I've only eaten at the Grand Floridian, so I'm not sure what Kona is like, but I loved the Grand Floridian when we went in January. I thought the choice of food and drink was really good. My brother asked for a drink of tea with his breakfast and was given a box full of Twinings tea, all different flavours. He was a bit over whelmed
I loved the character interaction as well. It was a very nice breakfast.