I am a longarm quilter so I hope this information is of help to you.
First I look at the intended use of the quilt, i.e. utility quilt, kid quilt to be drug around, wall hanging, etc. Then I consider the overall climate of where the quilt is going. (Even for wall hangings)
If it is to be a child's quilt, getting a lot of use I mostly use polyester batting. Poly stands up to multiple washings much better than cotton and doesn't require dense quilting.
If it is a bed quilt then I consider the climate for when the quilt will be used, i.e. winter only, always cool nights, etc. I will use a wool blend or Hobbs 80/20. Both of these have a good loft for stitch definition and breath so you don't get too hot or too cold. My decision maker is whether the quilt will be washed alot. If so, then I use Hobbs 80/20.
For a wall hanging I usually use 2 batts. I will use Warm n Natural for the bottom layer and usually a poly on top. Most wall hangings you want a design to stand out and that is the purpose for 2 layers of batt.
For a table runner and placemats I use Warm N Natural or Warm N White because it is flat and only do a basic quilting design on it.
I hope this helps.