Aristotle desired a reversal because of the suddenness of the impact, which heightens the emotional impact of the plot. As a constant element, the juxtaposition of opposites is less emotional and more intellectual in its effect. There are other details of both tragedies and epics that Aristotle identifies and describes in Poetics that could be used to draw a comparison between Oedipus Rex and the Odyssey, but even more effective for such a comparison are the direct statements Aristotle makes regarding the differences between epics and tragedies. Though Aristotle approves of epics and what they are able to accomplish and convey to their readers/hearers, he considers tragedy to be a higher form of art because it is necessarily more "refined" (Aristotle 62b2. Tragic plots are simpler and their stories shorter than epics, meaning the human emotion has been more finely reduced.

This can be seen very clearly be examining the plots and stories of Oedipus Rex and the Odyssey side by side. Though there are many characters and complexities in Oedipus Rex, and indeed the full story takes three plays to tell, the tragedy is boiled down to its essential elements, and is told in a direct and inevitable manner. The Odyssey, on the other hand, ranges far and wide...

The many different adventures that both Odysseus and Telemachus encounter are often related only by the fact that they happen to these characters, without any larger plot connections; the fact that the story has multiple characters that it follows and multiple themes and stories that it tries to resolve also show the greater complexity and inclusiveness of the work. Though this does not diminish the quality of the work itself according to Aristotle -- this complexity and broadness is, of course, exactly what an epic is meant to contain, and thus an epic that contains these more completely would be a better epic. Aristotle considers the tragedy to be a higher art form because it is able to elicit a more profound emotional response in a more direct and simplified manner, and this is definitely true of Oedipus Rex when compared to the Odyssey.
Both Homer and Sophocles were and are considered brilliant masters of Greek literature. Aristotle did not really proclaim one artist to be greater or lesser than the other; his comparison of art forms is largely academic and attempts to be subjective. The tragedy is a higher art form simply because it is more effective, and Aristotle does not limit qualitative aspects with the same scrutiny.