Ranka is basically a war orphan while Minmay lived with her aunt and uncle. But aside from the story differences, Ranka isn't the young Minmay. In that case, Minmay was young all the way until she and Kaifun broke up, then began to mature after Kaifun left. But then she just wants to be with Hikaru, something I don't see Sheryl would do. Sheryl has a huge ego too, and Minmay was never like that, even when she was famous.
Like I said in a previous post, Ranka is the anti-Minmay, but character wise, Ranka's own story makes sense. I don't like her because the writers have debased Minmay in the process when Ranka sang My Boyfriend is a Pilot in such a fashion, as well as dressing her up to be the new Minmay.
A another thread could be started on what is the best series, but I just say SDF:M is also liked because of it's mature characterizations and plot. I agree with the length and message of Macross 7, but in the end and after Encore and Dynamite, I liked it just as good as SDF: M. Macross F is filled with anime cliches, shaky plot, and a majority of characters you couldn't careless about (well me anyway). The animation is awesome, however. But that's all I'll say about this unless there's a topic somewhere about it.