The main and lead are kinda confusing to me too at first. ><"
but now based from "main" and "lead" in Kara and some groups, I think Lead is like leader of the song which means lead > main.
Though I like Gyuri & her voice the most,I have to admit that Gyuri's singing skill is still far behind from Seungyeon.(but yes...I still hope she will get much more line too...)