50wt 3-ply has been the standard thread sold in the U.S. for decades. However, it is far from my favorite thread for either piecing or quilting.
I have found that 50wt 2-ply Aurifil is the best so far for me in terms of piecing; its thinness helps the seam to iron out flat and therefore helps me keep my seams exact. Not all threads of the same weight and ply are created equal! Aurifil is a high quality cotton thread that is very smooth and non-linty. After reading a recent long thread about using cotton or polyester threads for piecing, I have finally decided to ditch poly and use cotton exclusively for piecing.
I also very much like 50wt 2-ply Aurifil for quilting on my domestic machine. A finer thread like this does not show my mistakes as much. Plus, I like to do a lot of quilting and this thread does not make my resulting quilt stiff. I have had a stiffness problem in the past quilting with standard 50wt 3-ply cotton thread.
For my midarm (Voyager 17), the 50wt 2-ply Aurifil is not strong enough. For frame quilting, my absolute favorite thread is Glide -- which is a 40wt polyester thread.
There is no "right" about FMQ; it's personal preference. There are quilters who use thick decorative threads for quilting too, in order to achieve the look they want.
Edit: I should add that you often cannot directly compare threads even if they have the same "weight" and ply. This is because "weight" has been redefined several times and now there really is no universal standard. There was an interesting thread (no pun intended!) about this problem several weeks ago.