The only way to decide is to read the subject for a few days. By doing so, you will see if it interests you or not. You don't need to buy any books, the books that are used the most are available in soft form( for Geo), so get them and spend a few days with them. Then you through the past papers. You'd know yourself which subject suits your temperament.

I am doing Geography. I found the paper 1 somewhat predictable.
About paper 2, doing it carefully would mean that you have prepared half of the Pak Affairs as well.

It is not very easy, but who said CSS is easy and any subject you pick will require some hard work.