Pretty much any sweetener will work in this recipe. Using liquid sweetener like maple syrup, honey or agave results in a really smooth consistency. Granulated sugar takes a little bit to dissolve but brown sugar dissolves pretty easily. There really is no wrong choice, it's more of a personal preference. Also, if you are looking to reduce the sugar content, the sweetener can be omitted. Just make sure you have a nice, ripe watermelon.