The same question poked my head last month. I thought about it from various angles and decided to opt for Political Science because of the following reasons:
1. It is easier to score in Political Science
2. Unlike Int'l Relations, Political Science is more descriptive than analytical
3. I opted for Int'l Relations in 2017 and got 100/200 (59 and 41), and believe that with a similar effort in Political Science, one can score 120+
4. Yes, Political Science is lengthy, but you have 8 months, which are enough