At what point do Mythics become stronger than Legendaries?

Just something I’m confused on. I’ve been using my legendaries as they’re all 2 to 3 blue stars, and my supes and Harley are only 3 regular stars, so I haven’t used them.

It was my understanding that having the extra abilities and having them levelled up meant legendaries would be better for the most part, but at what point does this stop?

Have I been wasting my Harley and superman, while putting too many resources into my legendaries?

Also, completely unrelated note, how do you work out which legacy item works best with who? I know there’s the guide showing you who should get what, but I wasn’t sure how accurate that was.