every subject is both high scoring and low scoring...it just depends upon your interest and the way you prepare it...perhaps the most important factor will be how you deal with your nerves and write ya brain + heart out in the paper....
the issue with any CSS paper is not that the candidates don't have the pre-requisite knowledge. it is the fact that they can't pen it down and hence they carry their ideas back to their homes locked in the brain while they were supposed to write em in the paper..
so think about your deltas...and en decide for the subjects... may God help you achieve what you really want to achieve
Sorry one more thing brother...Just like Current Affairs, Public Administration also deal with the different problems of bureaucracy etc etc in Pakistan these days. so most of the times you are helped with these topics by the daily newspapers....it is highly recommended that you go through the newspaper and watch talk shows and discussions etc...