@pleonasm - Well, one work of Juvenalian satire doesn't mean that the author is incapable of Horatian satire. By its very nature Juvenalian satire is going to be fairly specific and aimed at a particular segment of society, while Horatian is supposed to be making fun of humans as a whole.
In fact, I think that is the most important difference between them, rather than whether one type is more cruel than the other. Because if you are making fun of all people and their foibles, you are bringing people together and enhancing their similarities.
If you are making fun of a particular segment of society then you are separating people out, which can really make people upset.