It is tough to justify which one is better or more scoring as it depends up on your aptitude and intelligence.
I would like to add that none of the optional subject is low scoring for any candidate who is well prepared.
Philosophy and Sociology both are very good subjects. A lot of top ranks have been attained with both subjects.
Both Philosophy and Sociology render good help in Essay paper and in interview as well.
Sociology score over Philosophy on one big factor i.e. Success rate of Sociology is much higher than Philosophy.
Sociology extends more help in General Studies Paper I; while Philosophy has an edge with General Studies Paper IV (Ethics).
Many candidates achieve high scores with both these subjects; but, the fortunes of both the subjects changed in recent years.
While Sociology gained popularity and attracted large number of candidates in past few years and delivered large number of successes and a very good success-rate.
However, Philosophy has been going through the rough weather and has lost its sheen a little.
Other big differentiator is Philosophy is know to be favourite subject of Hindi Medium candidates where as Sociology is liked equally by both Hindi & English medium candidates.
The prime reason of Philosophy result going down - continuing bad performance of Hindi medium candidates. It's fortunes transformed after Main Examination pattern change with only ONE optional subject.
Yes, in English medium Philosophy is still performing and delivering some results.
For the first time in CSE 2010 result, some top ranks are visible with Philosophy in English medium which in turn surely impacted the choice of many candidates in future.
It has comparatively smaller syllabus and can be managed with in short span of 3-4 months.
Candidate from diverse background can take Philosophy.
The biggest advantage is that it helps in General Studies Paper IV (Ethics) preparation.
It is one among the most popular optional subjects that candidates are opting.
This subject is conceptual in nature and the division of topics is such that almost all are interlinked and with better understanding, it can be prepared in comparatively lesser time and effort.
As the Paper II is considered to be application of what you read in Paper I. So, on first look Sociology syllabus may look big but candidates find it easy to manage who understand this.
Sociology requires good writing/presentation skills which require clarity about concepts. Most of the concepts are easy and clear and there is not much of contradiction which makes it uncomplicated and majority of candidates are able to adapt the writing style Civil Services Examination requires.
What to Do?
You need to know how to deal the subject and if you able to meet specific demands any particular optional subjects throw, you will find it easy and will score higher marks.
So, be it Philosophy or Sociology, have a look at the syllabus and past year’s papers to know the requirements and current trends.
Make a realistic estimation based on your knowledge level, the amount of efforts you need to make to uplift your preparation level that matches examiner’s expectations.