Cotton, Polyester, Blended, Silk, Bamboo or Wool? How do you know which is right for you? There are several factors to weigh.
Price - If price is a concern you will find polyester the least expensive and wool the most expensive. Cotton is in between.
Warmth - Wool is the warmest followed by polyester then cotton. Cotton and wool both breathe better than polyester.
Drape - A lightweight batting will drape better than a thick batting. Polyester tends to drape better than cotton. If you are making a wallhanging that you don't want to sag on the wall, a thick cotton batting with hold its shape well. Wool drapes well and will hold its shape for wallhangings. If your quilt is folded wool is least apt to have any remaining creases.
Weight - If the quilt will be used as a cover, the weight is often a matter of personal preference. A baby's quilt should probably be less weighty than an adult's quilt. Wool and polyester are lighter weight than cotton. Most battings come in different weights for the same batting type.
Loft - If you want your quilt to look puffy then you'll want to use a wool or polyester batting. Among brands of wool and polyester you'll find that the same weight can have differing loft. Hobbs and Quilters Dream make some pretty lofty polyester battings.
Appearance - All of the Quilter's Dream batting I use has been pre-shrunk but there is still a little shrinkage to follow in cotton batting - especially if the quilt is washed in warm water and machine dried. Some people like the old-fashioned slightly puckered look you get from cotton batting when it shrinks a little more than the fabric. Polyester won't shrink. A blend will give you the benefits of the natural cotton fibers but the added polyester controls shrinkage so you won't get the slight puckering.
Environment - Cotton and wool are natural fibers however cotton growing is one of the more environmentally taxing processes. There are now greener batting options available. One option is Quilter's Dream Green which is made from recycled plastic bottles. It has the same feel as polyester and is around the same price Quilters Dream has also just released a new Orient batting made from silk, bamboo, Tencel and cotton. The feel is luxurious and it is made from more environmentally friendly products.
Quiltling - Hand quilters avoid battings that have obstacles to quilting in them. Natural cotton battings have some little seed bits in them making them more difficult to hand quilt. Different battings hold together differently. Some require more dense quilting than others to keep from separating or bunching. Make sure the batting you choose agrees with the kind of quilting you want to do.