Cotton batting makes it soft and warm, and polyester adds strength. I like to use the 80/20 cotton/polyester blend. It breaths, and yet is a little stronger than pure cotton. Plus I like to have it attached to a scrim. That's a lightweight backing that adds stability and strength to a quilt. You don't notice it's there. It's just good for the longevity and stability of the quilt. I've only gotten those at Nancy's Notions and it's on line. For smaller stuff, like a lap quilt, I tend to go for polyester, because it washes well and easily, and lap quilts tend to get dirty faster. You have to use decent fabric (nothing thin) with polyester or you'll end up with "bearding", where the fibers of the batting start sticking through the thin fabric. But don't worry about that for the 80/20 mix. So far I've not found it to be a problem. For the little stuff, like table runners, place mats, wall hangings, etc., I use the thinner, low loft cotton. There's a lot of variety out there. As one of the other members said, you've got to look around and see what's best for what you're making! If you want a really, really warm quilt, doubled flannel is great. If you're looking for a "summer quilt" that is very light weight, you can either sew your top right onto your backing. I've used quilts like that, and they're just fine. It's the way our Grandmothers used to do it because they didn't have batting. Ever wonder why they used to pile 7 or 10 quilts on a bed? Because they had no batting, or very little! Or use something very light weight in the middle of a Summer weight quilt, like muslin, so something similar. Look at all the choices you have!
Warmest Regards,