Aurifil is 50 wt 2 ply which is the most popular for piecing. Most other brands are 50 wt 3 ply. Masterpiece use to be 50 2 ply but was changed to 3 ply for strength. There are so many good brands of thread on the market now to choose.
I use 60 wt in the bobbin. I have been using 100 wt soft poly thread for machine applique and love it and for the WOW effect I use Sew Sassy 12 wt thread for machine applique. Using the 12 wt, the stitches look like hand embroidery.