The 50wt Aurifil is a 2-ply thread. Presencia 60wt is 3-ply. Strength is not simply a factor of weight, but also of ply. A fine thread can be just as strong or even stronger than a thick thread. I find both Aurifil and Presencia plenty strong enough for piecing, given a moderate amount of quilting. Quilting takes stress off the piecing seams. It may be that tied quilts and quilts with quilting lines far apart require a stronger piecing thread in order to take the additional stress without breaking.