Billingsley is the best bet, he wa the best pitching prospect in the minors and is pretty much a sure bet to stay up in the bigs. The next guy after Billingsley I'd take is Pelfrey as he is really stating to dominate AA. The Mets staff is probably going to need something else and instead of making a trade I think it will eventuall be Pelfrey.
Hughes was called Prior junior or lite for a few reasons. His stuff isn't as good as Prior's, a step below his or so(hence "lite" or "jr" the reference wasn't really made to him as Prior just younger). He has good control like him though and recently both have had injury concerns. I don't think he will be traded, I think the Yanks realize he is pretty close and with their aging starters could really use him in a few years. They can win and get in the play off race even being 2 OF's down.
Bailey in the best case scenario might get a Sept. call up. Remember he is still in High A, he would have to be promoted to AA (which can be a big transition) and then pretty much dominate there. He has the stuff/potential, but it's not a case like Cain. Matt Cain was in AAA the whole year and was pretty certain he wold get some time at the end of the year.