Mountain Mist low-loft poly is a fabulous batting to work with and it produces a very thin, light quilt. However, as I understand it, poly doesn't absorb and throw off the heat the way that a natural fiber does, such as cotton or wool, so it tends to be "hot" no matter how thin. It "seems" to be lighter weight and therefore not so warm to sit under, but that could be misleading.
Hopefully someone else here can confirm or deny what I've heard re poly batting.
I've never used flannel as a batting, but the more I hear about it from all you who have used it, the more interested I am in trying it out. :)