Frankly speaking, you have to prepare separately for UPSC and if you have a background to the subject then it will help you a little else there are no more advantages. Personally, I think Political Science will be better for you to study. Having a understanding in polity will help you as a administrator laterwards. But if you want to go for UPSC the you can choose any of it no matter what but the subject you really like and the subject which will not be a burden to you. Having a background in Geography will help you in a passive way as an administrator but having a depth knowledge of Political science will actively assist you in decision making as an administrator.

I hope this answer helps. All the very best for your future endeavors!