I used to use 50wt 3ply thread for piecing, but once I tried Aurifil 50wt 2ply thread I was hooked! It's finer and smoother, seams press better, and I have better accuracy using it. Plus there is a lot less lint to deal with it and the bobbin doesn't have to be changed as often (because the thread is finer, the bobbin holds more of it).
For me, the main reason to use Aurifil on top and 60wt bobbin thread on the bobbin is cost. Threadart 60wt poly bobbin thread is $6 for 5,000 meters. Aurifil costs about 3 to 4 times as much. My one concern is that the poly might eventually cut through the cotton. I know that is not supposed to be a concern any more, but when I test the poly it is *really* strong and I know cotton thread can dry out over time.
I may eventually bite the bullet and pay to use only great quality cotton thread but, since my current level of quilting is not what I would consider even near heirloom quality, I'm not worrying about it right now.
I love Aurifil 50wt 2ply for quilting on my domestic machine, but it will not be strong enough for my machine frame setup. The seller told me that 40wt Aurifil will work well in the top, but 50wt will break. I believe on the machine frame setups it's common to use a thicker thread on top and a finer thread in the bobbin.
My bet is that Masterpiece thread from Superior (which is touted as a great piecing thread) is very similar to Aurifil 50wt 2ply, but I have not tried Masterpiece so can't be sure.