Aristotle seems to treat tragedy and epic poetry as largely similar. They are both meant to be imitations of great deeds, noble heroes, and tragic suffering, the main difference being that tragedy conveys all this by means of action, while epic poetry does so by means of language alone. They deal in the same genre, so the only differences are determined by the different limitations imposed by the different media of expression. A tragedy, for instance, cannot be as long as an epic, nor can it portray so many different happenings or get away with as many fantastic events. On the other hand, tragedy is more focused, and epic poetry cannot make use of the music or spectacle of stage performances.
Tragedy, it seems, is a more realistic medium. Because we see everything in a tragedy happening before our eyes, the action is limited to the realm of human possibility. Admittedly, Greek stagecraft became increasingly complex, allowing actors to fly about suspended from cranes, and so on, but too much of this would be absurd. Indeed, Aristophanes, the great comic poet, made good comic use of such devices.
Epic poetry, on the other hand, is a purely narrative medium and as such is limited only by the imagination of the poet and listener. Because we have no help in visualizing events, the epic poet can more easily recount the improbable without disturbing us. Aristotle refers to the episode in the Iliad where Achilles chases Hector three times around the walls of Troy. Homer makes no mention of the rest of the Greek army, which presumably must have been sitting idly by, watching the chase. Such a picture would immediately seem ridiculous if presented on stage, but because Homer can focus exclusively on the characters of Achilles and Hector, we are liable not to notice this absurdity.
The larger-than-life qualities of epic poetry are also brought about by the heroic meter. This contrived and elevated meter further removes the characters in the story from realistic portrayal, their extraordinary speech meshing well with their extraordinary deeds. By contrast, tragedy employs an iambic meter that closely resembles the rhythms of everyday speech.
In spite of these differences, Aristotle seems to think that epic poetry and tragedy can be judged according to similar criteria. Most important to both is that they maintain unity of plot. Epic poetry, by virtue of its length, is more suited to episode and digression, but these digressions must be tied to the plot as tightly as the fewer digressions found in tragic poetry. Similar requirements regarding character presumably apply to the epic hero as to the tragic hero. In spite of the differences in genre, it would seem that the basic criteria for judging quality remain the same.