Are you testing the thread for piecing or quilting? Or both?
One other thing I can think of is how it looks ... I personally see a difference in the amount of "sheen" in two of the threads you have on your list.
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Another thing you should bear in mind, even if you are using all 50wt threads - not all 50wt's are the same. As an example 50wt Aurifil is a 2 ply and much thinner than a 50wt Connecting threads.
I use an 80/12 when I sew with Aurifil but I can't get away with that with Connecting Threads which is a much heavier 50wt. Sulky and King Tut I only buy in heavier weight so I can't compare. Gutterman 50wt I can also use an 80/12. Mettler I only buy in 60wt so I can't compare. The other brands I have not used.
Above needle sizes are all for piecing. My heavier weight threads I only use for quilting and all are different.
As for machines ... I think I would lean toward the 301, the 99 or the 15.