Diana Ross ballads vs Mary Wilson ballads...I don't think one was any better than the other once they matured. Mary's solo portion on "People" vs Diana's solo portion on "People, it's Mary hands down for me. She just fit right in there so perfectly in a way that I don't think Diana did. In fact, I've opined before that Mary doing "People" might have been a better fit than Florence doing it.

Diana singing "Come On Boy" or Mary singing "Can't Take My Eyes Off You" in Japan 73, or Diana singing "All the Befores" or Mary singing "Don't Let the Teardrops Bother You", they both for me were equally capable of selling a ballad.

The difference for me is the very early days. I think songs like "I Want a Guy" and "Who's Loving You" would have been ten times better with Mary on lead than Diana. I don't think very early Diana was a good ballad singer. Her voice seemed much more in control and commanding on faster songs. Smokey seemed to help her find her groove with "Your Heart Belongs To Me", but even that I have to wonder it maybe it would have done better on the charts with Mary rather than Diana.