I'm currently writing about Aristotle and Seneca on anger. The short of it is that Aristotle thinks that anger is sometimes rational (and to experience and express it sometimes virtuous) and Seneca denies this. Some of this disagreement stems from the larger disagreement between Aristotelians and Stoics about whether "externals" are necessary for a good life (and thus whether external misfortune truly harms oneself). But although Seneca shares Aristotle's definition of anger as the perception of a wrong to oneself (or another one cares about) and a desire for revenge, he stipulates a rather high threshold for what counts as being angry--which is that one must approve of the desire for revenge (in addition to affirming the perception of wrongdoing), at which point one becomes angry and is, as it were, done thinking. Once angry, anger won't listen further to reason. So, if you can control your "anger," then you aren't really angry on Seneca's view.
This might complicate the attempt to understand the extent of disagreement between Aristotle and Seneca. Perhaps all that can be said is that Aristotle thinks that it is right to approve of that desire for revenge (to deem it justified) and Seneca does not. But it also seems that, although Seneca says he more or less agrees with Aristotle's definition of anger, he actually doesn't, since real anger, for him, doesn't occur until one has assented to the angry impulse.
In a sense, then, there is an important difference for Seneca between feeling angry--he wouldn't call this anger in the strict sense--and being angry. I wonder whether this distinction might actually be helpful insofar as it presses us to distinguish between mere feelings (or impulses) and our judgments about them--the extent to which we identify with or reject them, in terms of whether we think it right to act on them in particular ways. We sometimes say, "I was angry with P," but what we really mean is that we were irritated, annoyed, frustrated, or so forth. And perhaps very much so. But we might have felt those things without deciding that we should exact some revenge upon P. And we might experience those feelings and still remain reasonable and in control in our dealings with P. Seneca would applaud us, and say that, in fact, we had not been angry. Had we become angry, we would have lost control.
I suppose if we experience an outburst--we start yelling and screaming, losing control even for a moment--then we crossed the threshold. So perhaps that kind of outburst should be counted as a kind of "revenge." In general, avoiding such outbursts, those momentary losses of control, seems like a good thing. (A calculated "outburst," would seem to be something different, and compatible with inner control.) And Seneca realizes that in order to do so, we probably also have to work on ourselves so that we are not too easily irritated, annoyed, etc. The feelings of anger can be hard to fight back. But again, Seneca would not say that we have to fight back our anger, but rather avoid becoming angry in the face of feelings that can provoke assent to anger. That is, anger doesn't happen to us, though feelings of irritation, annoyance, and frustration, etc., do. These are what the Stoics would call the "affective preludes" of anger.
My impression of the Stoic view is that it can be perfectly ok to experience the "affective preludes"--at least, there isn't much we can do, in the moment, about the impulses we experience except to respond to them ("deal with them"). And in a sense, this seems to leave room in the Stoic view for something we would normally call anger--namely, that cluster of feelings we associate with anger. The Sage, apparently, still has feelings, but he or she no longer responds to those feelings in the way that people generally do. Irritation no longer gives rise to anger. (And the Sage is presumably not irritated by many of the things that irritate ordinary people.) But the Sage is capable of feeling. This seems important, since critics of Stoicism claim that Stoicism lacks feeling. But what it lacks is emotion--understood specifically as false judgments. But feelings are something different from emotions on the Stoic view.
And so when Seneca says that anger is always to be avoided, he isn't saying that we should try to extirpate our capacity for the feelings we associate with anger--though if we are hot-tempered, we should probably work on that. (But Aristotle wouldn't disagree.) He's saying (roughly) that we shouldn't seek revenge. And when he says we shouldn't seek revenge, he's not saying that wrongdoers shouldn't be punished.* Rather, they should be dealt with fairly, reasonably. And that's very hard to do in the heat of anger.
* Perhaps the challenge here on the Stoic view is to work out what a "wrongdoer" is, since the Stoics don't regard loss of "externals" as a harm to the self. The only harm is loss of virtue, and you can only do that harm to yourself. But I suppose one could simply suggest that a wrongdoer is someone who abuses one's "preferred indifferents"--which includes one's physical security, possessions, as well as the other people one cares about...