Flannel is great for a backing...but preshrink. Friends of mine have used the Minkee too but that scares me to sew as it is slippery. My friend has had no problem with it though.
100% cotton just pieces really nice. It is easier to 'scootch' (also known as ease). I have used a little bit of polyester blend a few times where it was the only fitting color like for a face or a frame. It just does not handle or sew into pieces as nicely as 100% cotton.
Dot in Oregon