For me it all depends on the effect I am shooting for. Since I make all of my quilts on a treadle I purposely use 80/20 and never wash any of the materials or batting until it is finished and it gives my quilts the "vintage" look and feel to them after a trip through the washer and drier.
I use polyester on the baby quilts and the quilt for kids just because its easier on the little ones and possible allergies/skin issues.
But if I want a higher loft like the quilt I am making for my wife and I to put on our bed, I use the 3/4" thick polyester. It is really up to the person as to what batting to use. Just make a few quilts and use different battings and see what works well for you.
Billy