I personally have no doubt in my mind that in the end, we'll see Ranka "winning out" over Sheryl. It just fits the 'vibe' I get from the show thus far and from my understanding of anime plots thus far. I'd bet money on it, if I had money to bet. :P
I think that Ranka's character is not necessarily any less than Sheryls, rather it is simply the fact that they are two VERY different characters at very different stages in their lives. Sheryl is, in her life now, where Ranka -could- be eventually... yet in other says, Sheryl is less mature than Ranka. Right now we see Sheryl being fleshed out a lot- because the character did indeed seem rather shallow. But we are seeing seeds planted for the future development of Ranka as well. It is pretty clear to me that Ranka is the "lead female" here- she will be the key to everything that unfolds from here on out, and in the end I have a feeling some special power or unique past/background will develop with things only she can do. Likely it will be up to Alto to rescue her and bond with her in some unique way, and Sheryl will have to let him go.
Or have you guys not seen enough Animes with that exact same kind of plot with similar types of characters to get the same 'feeling' I have on this one? :P I felt from the first episode, and the second, and then on that it was clear Ranka was being set up as the one who would win Alto. If they do anything else, it will be a very sharp break from previous plot trends in most anime and quite a surprise- I welcome it if so, nice to get something 'different'... though I do kind of like Ranka over Sheryl.
PS- I think anyone who has seen some of the original Macross series (I didn't see it all- couldn't STAND the old style American dub... ick), but what I saw of it leads me to see a very clear parallel between Ranka's journey from normality to fame with the journey of the lead female in the original. It is, of course, a far deeper and more realistic journey- and I don't know if Ranka will make some of the same poor choices the lead in the original did, or if perhaps Sheryl embodies a bit more of someone who made those choices.