The wt and ply of a thread is more important then the type of thread. I think 50 wt 3ply thread is too heavy for piecing. Poly thread is stronger so it can be made thinner. Most top brands of piecing thread is poly. Most all bobbin thread used for bobbins is poly. I like cotton 2 ply 50 wt, like Aurifil for piecing. 70 wt poly for the bobbin. Superior threads has the best education site for threads and what needle size to use with each thread type.