generally machine quilting thread is thicker than piecing thread. Usually a 30wt thread..for piecing you should a 40 or 50wt..unless paper piecing, then it should be a 50 or 60wt. I try to use a 60wt in the bobbin for both!
The thicker the thread, the harder for your seams to lie flat!The thinner the thread the better the seams will lie down, especially at intersections!
I hope that helps!