IME, the biggest difference between the two disciplines is that most Pol. Sci. folks seek models for historical behavior. There are myriad theories (none of which I'm versed in) that guide the political scientist's research. Data are then accumulated and a model is either proposed or fitted to by the researcher.

Most historians, while nearly always guided by theory in one way or another, eschew models in favor of the microhistorical actor. Yes, it's possible that some person in some past place made a decision that fits within a model. But that person may well have chosen a different path -- one that does not fit any model. Does that person's decision mean less? Are their lives and decisions extraneous data? Most historians would answer in the negative.

As you might imagine, there is some silly intra-disciplinary humor that gets tossed around from time-to-time. Among the various jokes I've heard is that political scientists make for great friends. Their errors give us something to write about.