Hey,
It is completely your call.
UPSC is that field where the minimum eligibility criteria is to have a graduatiom degree in any stream which makes it the most favourable field. So irrespective of your discipline there is an equal chance of cracking the exam.
You should take Political Science only if you like the subject.
For Law,you can either do the five year integrated course or three year course after graduation in any field. Both of these are equally good and it is entirely your discretion.
Hope this helps.
Best of luck.