There is no real difference, except they are connected to each other. The concept allows basically the same food to be served from 2 different rooms, with different decor. To accommodate large numbers of people, RC wisely decided to have the 2, rather than one large Windjammer, their standard named restaurant. Hamburgers and other grilled foods are usually in the Island Grille. By the way, you have to pass through the Windjammer to access the Island Grille. You'll see what I mean after you are on board. Happy Sailing.
You have to go through the WINDJAMMER to get to the Island Grill. The latest serves eggs and omelettes to order for breakfast while the other doesn't. They both serve the same buffet for breakfast and lunch. The Island grill is also open in the evening for dinner (menu). We were on the Explorer last week and had a great time.