To me, a double layer of Warm and Natural is too hard to quilt through. I have used the 80/20 batting (80% cotton, 20% poly) and like it pretty well. One layer makes a lightweight quilt and a double layer is warmer and has more loft. With 2 layers I can still FMQ it on my domestic machine. I used Hobbs Washable Wool in one quilt and just love it--it's so warm and fluffy! The wool batting takes special care when washing though, while a quilt made with the 80/20 I can throw in the washer and dryer.
As far as trapunto goes, I've used poly batting so that it really puffs up, but it depends on the look you want.
I've seen several people mention that poly batting can "beard", but haven't seen that myself...My mom used poly batting in all her quilts and even after so much use that they are threadbare, they are not bearding. It does concern me that something like this could happen though...
The batting you choose use is entirely personal preference, and you'll find what you like. Right now JoAnn's has all of their batting at 50% off; you might buy some different varieties in the single packages so that you can try them out and find your favorite before committing to a whole roll of a single type.