If I were you, and you can afford it, I would use silk batting. I took a hand quilting class from Roxanne McElroy, and silk batting was her favorite. She passed around a quilt that had silk batting in it so we could feel it -- very light in weight. I believe silk is supposed to be cool in summer, warm in winter too. Roxanne lived in Hawaii and used her silk quilts there.
Second choice, if silk is too pricey, would be Hobbs wool batting.