Here's some info from Kaye Wood that just about sums it up:
More about batting!
If you are looking for the most common, easiest to work with batting, you can't go wrong with either a cotton batting, made of natural fibers, or a polyester batting, made of synthetic fibers.
Cotton is a very popular form of batting, because it is so versatile. On those shivery winter nights, cotton traps and absorbs the air, keeping you comfortably warm. However, it doesn't hold in the heat so when used in the summer, it will also keep you cooler. Cotton is thin in appearance and considered a low-loft batting, but it is heavier than polyester. Cotton batting also shifts and beards when you quilt with it, unless you get a bonded batting.
 Polyester on the other hand, can take wash after wash and still hold it's shape. Polyester batting is thicker, but the weight is still lighter than cotton batting. Polyester is great for keeping you warm without the bulky weight, but the fibers restrict air flow, so you can get very hot, very quickly! When you have to store your quilts, polyester is wonderful because it resists mold and mildew.
Now if you would like to utilize the qualities that both of these battings have to offer, try using a cotton/poly blend! You won't get the excess weight and your quilt will have all the benefits these fine fibers have to offer!