Dear Rabbi Kornfeld, The Gemara quotes Rav Aba (whom the gilyon says is Rav) that someone who says "moshia es yisrael" is a boor. Later in the same amud Rav Yosi Bar Yehuda says to say it. Isn't this is very unusual, that one amora should recommend saying something and another saying that someone who does so is a boor?
In addition to your question, we could also ask that Rebbi Yosi Bar Yehudah is not an Amora, but a Tana, whose opinion is cited in a Beraisa ("Tanu Rabanan..."). How, then, could Rav Aba argue on a Tana? (If Rav Aba is indeed Rav, then we know that Rav could argue with Tana'im. Alternatively, it could be that this Amora did not hear of the Beraisa of Rebbi Yosi Bar Yehudah.)
In truth, however, they are not arguing. Rav Aba did not say that one is not Yotzei if he says "Moshia Yisrael." He is Yotzei, but he is a Bor for using that text. Rebbi Yosi Bar Yehudah, similarly, does not say that one should say "Moshia Yisrael;" he merely says that one is Yotzei if he says it, but he will agree that one who says it is a Bor (see Background 49:2). This is what is meant by the Gemara that Rebbi Yosi Bar Yehudah means one may even ("Af") say "Moshia Yisrael."