I have been doing microstock for two months, got 3 extended sale all from SS, nothing from any other site I joined, I got $20 each, I would leave the option open on all sites, because each site attract different customers, so by eliminating all the lower paying ones, you are limiting your own exposure.
By the way, all my pictures at FT price at $50 each, so I will get 17.5 if ever I get a sale from them.