I use 50wt for piecing & sewing on the binding. You could use either a 40 or 50wt for the quilting. 40wt is thicker & shows up a bit more so it just depends on whether you want that or not.
Also, if you switch to 40wt thread, be sure to also switch your needle size accordingly. Pretty much all manufacturers tell you the recommended needle size for each weight. I know for Aurifil, 50/2 thread can take a 70/10 or an 80/12 needle. Aurifil 40/2 takes an 80/12 needle. Aurifil 28/2 (the heaviest thread you can run through a machine) takes a 90/14 needle.