Hi,
well, I would tell you firstly that I completed my bachelors in Political Science as a major subject and I.R as a minor subject.
Now, considering your question what I have analyzed is that I.R demands more analytical and discussion based approach while Political Science is a bit plain and easy as compared to I.R. I too considered opting it in CSS but did not go for it because of the analytical and technical demand of answers.
Tbh I adopted smart strategy for preparing Political Science by viewing the trend of questions in the Past Papers (but just in case of philosophers and constitutions). Besides that, I had recovered all the syllabus.
Political Science is a doable task obviously if you are determined. Good luck