As a parent, I’m all for saving money. I can’t answer whether an Oxy degree would be worth an extra $40,000. Typically graduate school is funded (assuming you are going for a PhD) so it’s not like med school where you’d be wise to save $40k to pay for med school.
Can you reach out to profs or the political science department head at each school and ask, for example, the percent of students that go on to graduate school and where? I do think you’re right about SLO being more STEM oriented, as it is a polytechnic university, while Oxy is more geared to the liberal arts.