Here's why I like Stan and don't like Peter:
In tonight's episode, Stan acted like a jerk, but he came from a sympathetic place. Before he started doing jerky things, we saw how upset he was that he thought he was losing his place. Of course it's ridiculous, but that's why it's funny. For him to go so far for such absurd reasoning makes it funny and is really perfectly in character for him. The same with Steve. He's usually very subservient, which is why whenever he sees a chance to improve his position he usually jumps on it with great enthusiasm, even if, again, it's for absurd reasons. This is also very in character for him, and why he's a likable character since he usually ends up, as you would expect, getting dumped on for his overconfidence. But it's done in a sympathetic way. Also, the way Stan and Steve go back and forth, with first Stan on top, then Steve, then Stan again, then finally Stan learning an obligatory, though relevant, lesson, makes it compelling to the viewer.
This is in contrast to Peter, who usually just does mean stuff to be mean, without any sympathy or dramatic narrative factor. Like, say, farting on Meg or killing Quagmire's cat or any of the thousand of other mean things he's done just for shock value.
My biggest laugh, though, was probably Francine doing her "experiment".