Thread weight is the most counter-intuitive number in quilting, because it runs backwards. A 50wt thread is thinner than a 40wt thread. The number describes how many kilometers of thread it takes to make a kilogram — so the more it takes, the finer the thread.
50wt for piecing
Piecing thread should disappear. Every seam you sew adds the thickness of the thread to the seam allowance, and a quilt block has a lot of seams. Thin thread keeps a scant quarter inch actually scant, which is what makes a twelve and a half inch block finish at twelve and a half.
50wt cotton is the standard for this, and long-staple cotton is worth the difference: fewer short fiber ends means less lint in the bobbin case and fewer breaks.
40wt for quilting
Quilting thread should be seen. It sits on top of the quilt and is the whole point of the pattern you spent an evening choosing. 40wt is heavier and shows the stitch without fighting the machine.
This is also where variegated and polyester threads earn their place — on the surface, where the color does work.
What about 60wt and 80wt?
Finer still. 60wt is used for applique and detailed piecing where bulk really matters, 80wt for English paper piecing and invisible machine applique, where the aim is for the stitch to vanish entirely.
The practical version
If you keep two threads in the house, make them a neutral 50wt for piecing and a 40wt in whatever color you are quilting with. That covers most of what a quilt needs.
Our thread is sorted by weight rather than by brand, because the weight is the decision.