I have a friend who (no joke) sent me an email this week asking me what I thought about the differences between Plato and Aristotle. He has often asserted that Plato and Aristotle differ in that Plato describes the 'why' of existence and Aristotle describes the 'how' of existence. I used to agree with him, but I'm starting to contrast Aristotle and Plato on different grounds.
The first issue of divergence is one we discussed in class last week; namely, Aristotle's pragmatic orientation vs Plato's idealism. From my reading in and outside of this class, I perceive Aristotle buying Plato's idea of the unmoved mover, causality, the eternal soul, etc, and then working his belief out in the real world. He sees the value of contemplation and philosophical inquiry, but he also acknowledges that a lot of the hoplites think of the Good materially.
The way he addresses eudaemonia in the Nicomachean Ethics and Rhetoric is indicative of the binary. In Ethics, Aristotle places emphasis on the idea that the highest human happiness is the contemplation of the Good. In Rhetoric, he defines happiness as good food, good healthy, good family, etc. At various points, he derides the masses for valuing/believing silly things, but he's also far more willing to 'play ball' in regard to what the people value (than Plato). He'll teach his students how to write a speech that caters to the young and wealthy, one that doesn't offend their sensibilities, or one that draws on popular maxims which are empirically untrue or contradictory to other maxims.
I see another large divergence between the two on the matter of style. A lot of Aristotle's writings are lecture notes, so they appear as lists. I imagine that in the course of his lecture he would draw warrants between his ideas, which would change meaning and draw his philosophy nearer to addressing causality. Also, he's far less likely than Plato to use grand analogy or metaphor in his writing, and when he does, the comparisons are between fairly concrete things. Plato, on the other hand, was writing to be read literarily as well as philosophically. He works hard to make his reader emotionally invest in his analogies (emotionally and visually). The emotional investment and magical lexicon make his writing seem more profound than Aristotle's dry, repetitive lists.
The final difference I see between the two is the kind of inquiry they two are willing to undertake. I know we tend to deride Aristotle as hegemonic, formal, and everything postmodernism despises, but I saw Rhetoric II and III through a different lens. Aristotle is also a rebel against the handbooks of his day; he's destroying standard forms instead of promoting them (like when he says the only real things necessary in an oration are claim and evidence). It seems that he has watched what people actually do and then drawn conclusions about their behavior and what affects them. In this way, I see him making a kind of naturalistic inquiry into the people of his present moment, kind of like the inquiry Erlandson proposes (without the Marxist/Freirean orientation). I don't see Plato or any of his characters taking such an open minded approach.