My two cents.
International Relations will always remain the popular opinion, since it complements other subjects. When I first went through the syllabus of both the subjects, Political Science seemed lengthier and harder than IR. You should analyze the syllabus to asses the difficulty level and also the amount of effort required.

Business Administration seems a logical choice given your background. But again, it has a lengthy syllabus and covers about 4 5 subjects studied under BBA. Look at the past papers and if you think they are solvable without substantial effort, then you have your answer.

History of Pakistan and India helps with the Pakistan Affairs paper. Though, this year it was of no help.

Sociology has been the general choice among aspirants. But regional languages usually fetch better scores. Whichever among the two you think you can master in the given time will be best.

As for the remaining subject, with IR , Int'l Law is suggested, and with Political Science Constitutional Law, since you'll already be studying a major portion of these in the 200 marks subject.

As for the notion of substantial effort, CSS requires blood and sweat, but the balance of effort on each subject is necessary, so as all the subjects get equal effort. This means going through the syllabus, calculating the time and effort required, and then deciding whether they will interfere with the rest of the subjects. This comes from personal experience, I gave importance to one subject at the expense of another, and I regret it immensely.

Best of luck.