Cotton batting will hold a crease longer than a wool batting will, because the wool has more spring to it so it bounces back. I much prefer the wool to the cotton because I think quilts drape nicer with wool and it shows off quilting much better. It's also pretty breathable! It is more expensive than cotton, but for me it's worth it to not get hard creases in my quilts every time I unfold them.
|It can be hard to see in a photo, but see that crease right through the middle flowers?|
Watch my little tutorial on cotton vs. wool batting: