I'm currently reading Tom Holmén's entry, "Authenticity Criteria," which appears in the Encyclopedia of the Historical Jesus (edited by Craig A. Evans; New York: Routledge, 2008, 43–54). The piece is frustrating, at least to me, because Holmén continues to advocate an atomistic approach to reconstructing the "historical Jesus." In this approach, a critic will take an individual piece of the tradition (e.g., Luke 11.20 par.) and run it through a battery of tests. If the piece passes the battery, it then features in his historical reconstruction. If not, the piece is discarded.
But things are even more complicated than this. Holmén also advocates identifying and, again, discarding secondary additions to the tradition (e.g., many scholars, but not Holmén in this context, view the phrase "in spirit" in Matt. 5.3 as a later addition to something Jesus probably did say, which is more pristinely preserved in Luke 6.20). While this sounds good (who wouldn't want to identify corruptions in the tradition and remove them?!), in practice NT scholars have never been able to pull this off. The problem may be with NT scholars (we're just not smart enough to pull this off, or more charitably, perhaps not enough evidence has survived for us to distill authentic and/or original material from latter additions). But I think it more likely that the problem lies with the approach Holmén and traditional NT scholarship have advocated.
Instead of putting two or more parallel passages beside one another and arguing that this or that feature of one of the passages is later and secondary (what NT scholars call "tradition criticism"), NT scholars need to learn to accept the multiformity that lies at the heart of the Jesus tradition and to stop trying to reduce the extant multiforms to a single original forebear. For one thing, nothing gives us the right to suppose that Jesus ever said anything only once (even such striking and context-bound sayings as, "Give to Caesar . . ." or "Let the dead bury the dead . . ." may have been said on multiple occasions). But for another, what we have preserved for us in the Gospels are not more-or-less original traditions and the corruption of those traditions in later texts. The multiforms provide, instead, stereoscopic access for us to see the types of things Jesus' followers could say about him that (i) made sense of Jesus to themselves, (ii) made Jesus relevant and applicable in later situations, and (iii) illuminated appropriate and desirable courses of actions in the face of new and challenging questions.
So did Jesus say, "Blessed are you poor," as Luke has it, or "Blessed are the poor in spirit," as we read in Matthew? Perhaps both, perhaps neither. As a historian, I simply cannot prove that one or the other or either of them accurately records words that Jesus spoke on any given occasion. But what we do have preserved in the Gospels are two authors who attribute to Jesus strikingly similar (but not identical) sayings, and both seem to understand Jesus in similar (but, again, not identical) ways. This is exactly what we ought to expect, again, as historians, if the authors of Luke and Matthew had any access, direct or indirect, to the real Jesus.
More importantly, their portrayals of Jesus speaking his blessing on the poor (in spirit?) were sufficiently plausible to (i) be accepted, (ii) preserved, and (iii) disseminated. If Jesus was not the kind of person to bless the poor, spiritual or otherwise, such portrayals should have been less plausible in the first century. If Jesus was actually a friend of the rich and an elitist with regard to the poor, our extant sources have simply forgotten too much of the truth for us to know it. But if he actually pronounced blessing on the poor and preferred the socially marginalized, then our sources have preserved exactly this image of Jesus, and we are able to know something about this Jesus on the basis of their testimony. This is a significant (though not new) conclusion of historical scholarship on Jesus. But none of it, I would stress, depends on identifying secondary or later additions to the tradition and removing them. Such historical-critical approaches are, I think, well beyond their sell-by date.