I would use a batting that can take a lot of washing, so I would go with some king of polyester...and depending on how your finances are they are less expensive than cotton or cotton blends. And it also depends on how many you make. I worked for a non-profit that made comforters for charity for overseas use and local disasters, and used a 12 oz. Hobbs poly but these were tied, and you say you longarm them they would probably be too "puffy" for the machine. I would probably go with Quilters Dream poly, they have a good selection.