Wool batting is more expensive than cotton batting, but not prohibitively expensive. Silk batting is also wonderful for hand quilting, but it is a lot more expensive than wool. Silk would be nice for a really special quilt, as it is so light in weight.
With wool, it's important to stick with a good name brand. Hobbs wool and Quilter's Dream wool are the best, as far as I know. These battings are manufactured so that they react in a very similar manner to cotton when washed.