Go for ES. According to scoring trend it is more scoring and one can score above 90 in it while GS is medium scorer subject. Syllabus of ES is not very technical and one can grasp the subject easily in 15-20 days. It also helps in GSA. It can easily be prepared from 1 book. GS is a very technical subject. 1st you are required to get familiarity about key terms than you start preparation. No good local book available except Caravan book to some extent. Also you need to consult 3-4 books and reports for preparing well. The only advantage ES has is it is very interesting. About GS helping in Essay and Islamic Studies, yes it helps but contribution is very small and only in women related topics.
Rest it depends upon your interest and aptitude.