At Parrot Cay you get soda for no extra charge. At topsiders' if you want it, you need to go to one of the bars and buy it (someone posted that they sometimes sell cans at the end of the line, but I've never seen it.)
Also, most Topsiders' seating is outside, while Parrot Cay is inside. If it's rainy, chilly, or windy, that can make a difference.