Murphy's attitude on The Glee Project, and various comments from him regarding the conception of Glee in the first place, suggests that he's strong on taking the ACTUAL backstories of the actors and injecting it into their stories versus coming up with something on his own (or if you will, that he's a lazy writer). In any case, while Marissa is an obvious talent, I think that compared to lots of the others she wouldn't have a character that Murphy could write to. She looks great and can sing, but what's in her background that he could use? He wouldn't have known of her anorexia. Given the background, he would have axed her before the two guys he could potentially write to.
It thus becomes a case of Murphy and co. finding someone of the contestants whose story could become a character on the show, AND making sure that the actor who win will be physically and mentally capable of portraying that character for the episodes they'll be in. McKinleigh was a strong performer who a background in country music that Murphy could have written; however her attitude as an actor is what got her out. Likewise, Matteus had a talent, not to mention a unique physical look that could have fit well for a character, but there's no way he would have been mature enough to handle a seven episode stint, so he's out too. Alex (cross dressing black guy but a total diva) and Cameron (great look and talent but can't handle a shoot mentally or physically) would make great characters, but I doubt they'll last either because they would not be able to handle the actual shooting of the show. This aspect has not been focused on in the competition, but will be figuring into ANY decision to hand a seven-episode commitment to an unknown actor on a major network show.
My money is on dreadlocks guy or Rachel-clone. They have talent, can handle the environment, and can be written for (the latter as a true Rachel replacement, the former as, well, a dreadlocks guy). Big redhead has the right attitude but probably lacks the physical ability to do it, and Irish guy can hack the shoot but like Marissa is too generic IMO to write for, and I don't see Murphy sparing him once they run out of people with ACTUAL problems to axe instead of him. Or, Murphy has remembered the "Jesse's Girl" mixup a few episodes back and would write Irish guy as a gender-confused person - as if we needed more of that on this show. :P
Mark