Kona is one of my top three choices for WDW restaurants!!! I love them for breakfast, lunch or dinner- excellent, fresh, creative food- an one of the best values in WDW.
Haven't tried Garden Grill, but it's a completely different set-up and atmosphere- GG has characters and a family style service of American foods, while Kona is a relaxed Polynesian cafe atmosphere with a menu that concentrates on incorporating fresh fruits, vegetables and seafood in creative ways (though there are burgers at lunch, and pasta, stir fry, chicken, etc. at dinner) Desserts at Kona are fab!
Look at the menus and the pictures on the "Post your Disney Dining Pics Here" thread. They may help you decide. However, if you've never done Kona, I vote KONA!