Hello.. I am no expert, but I can tell you what I do. I usually do use the same thread for quilting and piecing. I have Gutterman, Aurafill, Superior and King Tut. I can say that I nearly exclusively use King Tut to quilt... but I have used it to piece as well.
I personally don't believe all fabric is equal. I have purchased the cheaper fabric but what I have experienced is that it is often thin. I have actually even double layered fabric because it was so thin that the color was pale.
I don't think its always the price, sometimes I have to feel the fabric.
You can find lots of info on YouTube and Google
Have fun