There's always a dilemma in debating any issue as to whether one should rely on emotion or reason to advance one's argument. You'll find this dilemma surrounding issues such as the death penalty, abortion and gay marriage. And it's especially the case in animal rights advocacy.
Many people say that PETA relies too heavily on emotion to argue its position. Others say that emotion is the only way to move humans to care about anything other than themselves.
I'm really torn. I think you have to take a little of A and a little of B. I have a terrible temper and can throw out the vitriol with the best of them. I can pack an emotional punch as well. But in my experience, it usually backfires, particularly with men. One comes across as the hysterical female who can't reason her position. This is frustrating on many different levels. On the other hand, clear cold reasoning can fall flat if there is no conviction underlying the words.
I think the best thing to do first is educate yourself about the issues. If you know what you are talking about, then you won't get flustered and angry. You know your points and you lay them out. But I don't think there's anything wrong with indignation either. It just has to be backed up with facts, reasoning and logic. The combination is a powerful weapon for advocacy.